diff --git a/src/Conversion/ONNXToKrnl/CMakeLists.txt b/src/Conversion/ONNXToKrnl/CMakeLists.txt
index 7684649..bab2dd3 100644
--- a/src/Conversion/ONNXToKrnl/CMakeLists.txt
+++ b/src/Conversion/ONNXToKrnl/CMakeLists.txt
@@ -24,6 +24,7 @@ add_library(OMONNXToKrnl
         Tensor/Concat.cpp
         Tensor/Split.cpp
         Tensor/Gather.cpp
+        Tensor/Size.cpp
         ConvertONNXToKrnl.cpp)
 target_link_libraries(OMONNXToKrnl
         onnx)
diff --git a/src/Conversion/ONNXToKrnl/ConvertONNXToKrnl.cpp b/src/Conversion/ONNXToKrnl/ConvertONNXToKrnl.cpp
index 083202f..a559c40 100644
--- a/src/Conversion/ONNXToKrnl/ConvertONNXToKrnl.cpp
+++ b/src/Conversion/ONNXToKrnl/ConvertONNXToKrnl.cpp
@@ -103,6 +103,7 @@ void FrontendToKrnlLoweringPass::runOnOperation() {
   populateLoweringONNXConcatOpPattern(patterns, &getContext());
   populateLoweringONNXSqueezeOpPattern(patterns, &getContext());
   populateLoweringONNXSplitOpPattern(patterns, &getContext());
+  populateLoweringONNXSizeOpPattern(patterns, &getContext());
   // Neural network
   populateLoweringONNXConvOpPattern(patterns, &getContext());
   populateLoweringONNXNormalizationOpPattern(patterns, &getContext());
diff --git a/src/Conversion/ONNXToKrnl/ONNXToKrnlCommon.hpp b/src/Conversion/ONNXToKrnl/ONNXToKrnlCommon.hpp
index d39012e..aa3d848 100644
--- a/src/Conversion/ONNXToKrnl/ONNXToKrnlCommon.hpp
+++ b/src/Conversion/ONNXToKrnl/ONNXToKrnlCommon.hpp
@@ -253,6 +253,9 @@ void populateLoweringONNXSqueezeOpPattern(
 void populateLoweringONNXSplitOpPattern(
     OwningRewritePatternList &patterns, MLIRContext *ctx);
 
+void populateLoweringONNXSizeOpPattern(
+    OwningRewritePatternList &patterns, MLIRContext *ctx);
+
 bool checkOpResultIsUsedByGetRef(AllocOp *allocOp);
 
 int64_t getMemRefSizeInBytes(Value val);
diff --git a/src/Conversion/ONNXToKrnl/Tensor/Size.cpp b/src/Conversion/ONNXToKrnl/Tensor/Size.cpp
new file mode 100644
index 0000000..fff208b
--- /dev/null
+++ b/src/Conversion/ONNXToKrnl/Tensor/Size.cpp
@@ -0,0 +1,73 @@
+//===---------------- Size.cpp - Lowering Size Op
+//-------------------===//
+//
+// Copyright 2019 The IBM Research Authors.
+//
+// =============================================================================
+//
+// This file lowers the ONNX Size Operator to Krnl dialect.
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/Conversion/ONNXToKrnl/ONNXToKrnlCommon.hpp"
+
+using namespace mlir;
+
+struct ONNXSizeOpLowering : public ConversionPattern {
+  ONNXSizeOpLowering(MLIRContext *ctx)
+      : ConversionPattern(mlir::ONNXSizeOp::getOperationName(), 1, ctx) {}
+
+  LogicalResult matchAndRewrite(Operation *op, ArrayRef<Value> operands,
+      ConversionPatternRewriter &rewriter) const final {
+    // Gather info.
+    Location loc = op->getLoc();
+    Value alloc;
+    bool insertDealloc = checkInsertDealloc(op);
+    ONNXSizeOp sizeOp = llvm::dyn_cast<ONNXSizeOp>(op);
+
+    ONNXSizeOpAdaptor operandAdaptor(operands);
+    Value data = operandAdaptor.data();
+    ArrayRef<int64_t> dataShape = data.getType().cast<MemRefType>().getShape();
+    Value resultOperand = sizeOp.size();
+    ValueRange indices;
+    MemRefType memRefType = convertToMemRefType(*op->result_type_begin());
+
+    if (hasAllConstantDimensions(memRefType))
+      alloc = insertAllocAndDealloc(memRefType, loc, rewriter, insertDealloc);
+    else
+      alloc = insertAllocAndDealloc(
+          memRefType, loc, rewriter, insertDealloc, {resultOperand});
+
+    // Accumulate static dimensions first.
+    int64_t staticNumElement = 1;
+    bool allStaticDimensions = true;
+    for (unsigned i = 0; i < dataShape.size(); i++) {
+      if (dataShape[i] != -1)
+        staticNumElement *= dataShape[i];
+      else
+        allStaticDimensions = false;
+    }
+    // Accumulate the remaining dimensions that are unknown.
+    Value noElements = emitConstantOp(
+        rewriter, loc, memRefType.getElementType(), staticNumElement);
+    if (!allStaticDimensions) {
+      for (unsigned i = 0; i < dataShape.size(); i++) {
+        if (dataShape[i] == -1) {
+          Value index = rewriter.create<DimOp>(loc, data, i);
+          Value dim = rewriter.create<IndexCastOp>(
+              loc, index, memRefType.getElementType());
+          noElements = rewriter.create<MulIOp>(loc, noElements, dim);
+        }
+      }
+    }
+
+    rewriter.create<AffineStoreOp>(loc, noElements, alloc, llvm::None);
+    rewriter.replaceOp(op, alloc);
+    return success();
+  }
+};
+
+void populateLoweringONNXSizeOpPattern(
+    OwningRewritePatternList &patterns, MLIRContext *ctx) {
+  patterns.insert<ONNXSizeOpLowering>(ctx);
+}
diff --git a/test/backend/test.py b/test/backend/test.py
index 94c5523..a25f891 100644
--- a/test/backend/test.py
+++ b/test/backend/test.py
@@ -422,6 +422,12 @@ test_to_enable = [
 
     # ConstantOfShape
     "test_constantofshape_float_ones_cpu",
+    
+    # Size
+    # TODO(tjingrant): fix unit test for size ops.
+    # "test_size_cpu",
+    # "test_size_example_cpu",
+    
     # Error:
     #    Items are not equal:
     #     ACTUAL: dtype('int32')
diff --git a/test/mlir/onnx/onnx_lowering.mlir b/test/mlir/onnx/onnx_lowering.mlir
index 6519a42..0882f0d 100644
--- a/test/mlir/onnx/onnx_lowering.mlir
+++ b/test/mlir/onnx/onnx_lowering.mlir
@@ -2135,6 +2135,42 @@ func @cast_lowering_f64f32_10(%arg0: tensor<10xf64>) -> tensor<*xf32> {
 
 // -----
 
+func @test_size_known(%arg0: tensor<2x2xf32>) -> tensor<i64> {
+  %1 = "onnx.Size"(%arg0) : (tensor<2x2xf32>) -> tensor<i64>
+  "std.return"(%1) : (tensor<i64>) -> ()
+
+  // CHECK-LABEL: test_size_known
+  // CHECK:      [[RES:%.+]] = alloc() : memref<i64>
+  // CHECK-NEXT  [[SIZE:%.+]] = constant 4 : i64
+  // CHECK-NEXT  affine.store [[SIZE]], [[RES]][] : memref<i64>
+  // CHECK-NEXT  return [[RES]] : memref<i64>
+
+}
+
+// -----
+
+func @test_size_unknown(%arg0 : tensor<?x2x?xf32>) -> tensor<i64> {
+
+  // CHECK-LABEL: test_size_unknown
+  // CHECK:       [[RES:%.+]] = alloc() : memref<i64>
+  // CHECK-NEXT:  [[INIT:%.+]] = constant 2 : i64
+  // CHECK-NEXT:  [[IND1:%.+]] = constant 0 : index
+  // CHECK-NEXT:  [[DIM1:%.+]] = dim %arg0, [[IND1]] : memref<?x2x?xf32>
+  // CHECK-NEXT:  [[CAST1:%.+]] = index_cast [[DIM1]] : index to i64
+  // CHECK-NEXT:  [[TMP1:%.+]] = muli [[INIT]], [[CAST1]] : i64
+  // CHECK-NEXT:  [[IND2:%.+]] = constant 2 : index
+  // CHECK-NEXT:  [[DIM2:%.+]] = dim %arg0, [[IND2]] : memref<?x2x?xf32>
+  // CHECK-NEXT:  [[IND3:%.+]] = index_cast [[DIM2]] : index to i64
+  // CHECK-NEXT:  [[SIZE:%.+]] = muli [[TMP1]], [[IND3]] : i64
+  // CHECK-NEXT:  affine.store [[SIZE]], [[RES]][] : memref<i64>
+  // CHECK-NEXT:  return [[RES]] : memref<i64>
+
+  %1 = "onnx.Size"(%arg0)  : (tensor<?x2x?xf32>) -> tensor<i64>
+  "std.return"(%1) : (tensor<i64>) -> ()
+}
+
+// -----
+
 // Test gather along axis 0, first example in ONNX for Gather.
 func @test_gather_axis0(%arg0 : tensor<3x2xf32>) -> tensor<2x2x2xf32> {
   %indices = "onnx.Constant"() {value = dense<[[0, 1], [1, 2]]> : tensor<2x2xi64>} : () -> tensor<2x2xi64>