diff --git a/src/Microsoft.ML.LightGbm/LightGbmArguments.cs b/src/Microsoft.ML.LightGbm/LightGbmArguments.cs
index a05d57fd1b..765b0d297a 100644
--- a/src/Microsoft.ML.LightGbm/LightGbmArguments.cs
+++ b/src/Microsoft.ML.LightGbm/LightGbmArguments.cs
@@ -58,7 +58,6 @@ public BoosterParameterBase(OptionsBase options)
         public abstract class OptionsBase : IBoosterParameterFactory
         {
             internal BoosterParameterBase GetBooster() { return null; }
-
             /// <summary>
             /// The minimum loss reduction required to make a further partition on a leaf node of the tree.
             /// </summary>
diff --git a/src/Microsoft.ML.LightGbm/LightGbmMulticlassTrainer.cs b/src/Microsoft.ML.LightGbm/LightGbmMulticlassTrainer.cs
index f82b937a14..353856e7bd 100644
--- a/src/Microsoft.ML.LightGbm/LightGbmMulticlassTrainer.cs
+++ b/src/Microsoft.ML.LightGbm/LightGbmMulticlassTrainer.cs
@@ -75,6 +75,12 @@ public enum EvaluateMetricType
                 LogLoss,
             }
 
+            /// <summary>
+            /// Whether training data is unbalanced.
+            /// </summary>
+            [Argument(ArgumentType.AtMostOnce, HelpText = "Use for multi-class classification when training data is not balanced", ShortName = "us")]
+            public bool UnbalancedSets = false;
+
             /// <summary>
             /// Whether to use softmax loss.
             /// </summary>
@@ -110,6 +116,7 @@ internal override Dictionary<string, object> ToDictionary(IHost host)
             {
                 var res = base.ToDictionary(host);
 
+                res[GetOptionName(nameof(UnbalancedSets))] = UnbalancedSets;
                 res[GetOptionName(nameof(Sigmoid))] = Sigmoid;
                 res[GetOptionName(nameof(EvaluateMetricType))] = GetOptionName(EvaluationMetric.ToString());
 
diff --git a/test/BaselineOutput/Common/EntryPoints/core_manifest.json b/test/BaselineOutput/Common/EntryPoints/core_manifest.json
index b7ea7b1534..c5a734b6b3 100644
--- a/test/BaselineOutput/Common/EntryPoints/core_manifest.json
+++ b/test/BaselineOutput/Common/EntryPoints/core_manifest.json
@@ -11974,6 +11974,18 @@
           "IsNullable": false,
           "Default": "Auto"
         },
+        {
+          "Name": "UnbalancedSets",
+          "Type": "Bool",
+          "Desc": "Use for multi-class classification when training data is not balanced",
+          "Aliases": [
+            "us"
+          ],
+          "Required": false,
+          "SortOrder": 150.0,
+          "IsNullable": false,
+          "Default": false
+        },
         {
           "Name": "UseSoftmax",
           "Type": "Bool",
diff --git a/test/Microsoft.ML.Tests/TrainerEstimators/TreeEnsembleFeaturizerTest.cs b/test/Microsoft.ML.Tests/TrainerEstimators/TreeEnsembleFeaturizerTest.cs
index ea8026f99e..71253a32b3 100644
--- a/test/Microsoft.ML.Tests/TrainerEstimators/TreeEnsembleFeaturizerTest.cs
+++ b/test/Microsoft.ML.Tests/TrainerEstimators/TreeEnsembleFeaturizerTest.cs
@@ -812,7 +812,7 @@ public void TreeEnsembleFeaturizingPipelineMulticlass()
 
         private class RowWithKey
         {
-            [KeyType()]
+            [KeyType(4)]
             public uint KeyLabel { get; set; }
         }
 
diff --git a/test/Microsoft.ML.Tests/TrainerEstimators/TreeEstimators.cs b/test/Microsoft.ML.Tests/TrainerEstimators/TreeEstimators.cs
index 73a5fa4149..b95f84b9a6 100644
--- a/test/Microsoft.ML.Tests/TrainerEstimators/TreeEstimators.cs
+++ b/test/Microsoft.ML.Tests/TrainerEstimators/TreeEstimators.cs
@@ -55,6 +55,28 @@ public void LightGBMBinaryEstimator()
                 NumberOfLeaves = 10,
                 NumberOfThreads = 1,
                 MinimumExampleCountPerLeaf = 2,
+                UnbalancedSets = false, // default value
+            });
+
+            var pipeWithTrainer = pipe.Append(trainer);
+            TestEstimatorCore(pipeWithTrainer, dataView);
+
+            var transformedDataView = pipe.Fit(dataView).Transform(dataView);
+            var model = trainer.Fit(transformedDataView, transformedDataView);
+            Done();
+        }
+
+        [LightGBMFact]
+        public void LightGBMBinaryEstimatorUnbalanced()
+        {
+            var (pipe, dataView) = GetBinaryClassificationPipeline();
+
+            var trainer = ML.BinaryClassification.Trainers.LightGbm(new LightGbmBinaryTrainer.Options
+            {
+                NumberOfLeaves = 10,
+                NumberOfThreads = 1,
+                MinimumExampleCountPerLeaf = 2,
+                UnbalancedSets = true,
             });
 
             var pipeWithTrainer = pipe.Append(trainer);
@@ -322,6 +344,44 @@ public void LightGbmMulticlassEstimatorCorrectSigmoid()
             Done();
         }
 
+        /// <summary>
+        /// LightGbmMulticlass Test of Balanced Data
+        /// </summary>
+        [LightGBMFact]
+        public void LightGbmMulticlassEstimatorBalanced()
+        {
+            var (pipeline, dataView) = GetMulticlassPipeline();
+
+            var trainer = ML.MulticlassClassification.Trainers.LightGbm(new LightGbmMulticlassTrainer.Options
+            {
+                UnbalancedSets = false
+            });
+
+            var pipe = pipeline.Append(trainer)
+                    .Append(new KeyToValueMappingEstimator(Env, "PredictedLabel"));
+            TestEstimatorCore(pipe, dataView);
+            Done();
+        }
+
+        /// <summary>
+        /// LightGbmMulticlass Test of Unbalanced Data
+        /// </summary>
+        [LightGBMFact]
+        public void LightGbmMulticlassEstimatorUnbalanced()
+        {
+            var (pipeline, dataView) = GetMulticlassPipeline();
+
+            var trainer = ML.MulticlassClassification.Trainers.LightGbm(new LightGbmMulticlassTrainer.Options
+            {
+                UnbalancedSets = true
+            });
+
+            var pipe = pipeline.Append(trainer)
+                    .Append(new KeyToValueMappingEstimator(Env, "PredictedLabel"));
+            TestEstimatorCore(pipe, dataView);
+            Done();
+        }
+
         // Number of examples
         private const int _rowNumber = 1000;
         // Number of features
@@ -338,7 +398,7 @@ private class GbmExample
             public float[] Score;
         }
 
-        private void LightGbmHelper(bool useSoftmax, double sigmoid, out string modelString, out List<GbmExample> mlnetPredictions, out double[] lgbmRawScores, out double[] lgbmProbabilities)
+        private void LightGbmHelper(bool useSoftmax, double sigmoid, out string modelString, out List<GbmExample> mlnetPredictions, out double[] lgbmRawScores, out double[] lgbmProbabilities, bool unbalancedSets = false)
         {
             // Prepare data and train LightGBM model via ML.NET
             // Training matrix. It contains all feature vectors.
@@ -372,7 +432,8 @@ private void LightGbmHelper(bool useSoftmax, double sigmoid, out string modelStr
                     MinimumExampleCountPerGroup = 1,
                     MinimumExampleCountPerLeaf = 1,
                     UseSoftmax = useSoftmax,
-                    Sigmoid = sigmoid // Custom sigmoid value.
+                    Sigmoid = sigmoid, // Custom sigmoid value.
+                    UnbalancedSets = unbalancedSets // false by default
                 });
 
             var gbm = gbmTrainer.Fit(dataView);
@@ -583,6 +644,35 @@ public void LightGbmMulticlassEstimatorCompareSoftMax()
             Done();
         }
 
+        [LightGBMFact]
+        public void LightGbmMulticlassEstimatorCompareUnbalanced()
+        {
+            // Train ML.NET LightGBM and native LightGBM and apply the trained models to the training set.
+            LightGbmHelper(useSoftmax: true, sigmoid: .5, out string modelString, out List<GbmExample> mlnetPredictions, out double[] nativeResult1, out double[] nativeResult0, unbalancedSets:true);
+
+            // The i-th predictor returned by LightGBM produces the raw score, denoted by z_i, of the i-th class.
+            // Assume that we have n classes in total. The i-th class probability can be computed via
+            // p_i = exp(z_i) / (exp(z_1) + ... + exp(z_n)).
+            Assert.True(modelString != null);
+            // Compare native LightGBM's and ML.NET's LightGBM results example by example
+            for (int i = 0; i < _rowNumber; ++i)
+            {
+                double sum = 0;
+                for (int j = 0; j < _classNumber; ++j)
+                {
+                    Assert.Equal(nativeResult0[j + i * _classNumber], mlnetPredictions[i].Score[j], 6);
+                    sum += Math.Exp((float)nativeResult1[j + i * _classNumber]);
+                }
+                for (int j = 0; j < _classNumber; ++j)
+                {
+                    double prob = Math.Exp(nativeResult1[j + i * _classNumber]);
+                    Assert.Equal(prob / sum, mlnetPredictions[i].Score[j], 6);
+                }
+            }
+
+            Done();
+        }
+
         [LightGBMFact]
         public void LightGbmInDifferentCulture()
         {