diff --git a/lib/backends/cpu/ops-resolve.ts b/lib/backends/cpu/ops-resolve.ts
index 8b50e1d5..71793ffb 100644
--- a/lib/backends/cpu/ops-resolve.ts
+++ b/lib/backends/cpu/ops-resolve.ts
@@ -10,6 +10,7 @@ import {CpuBinaryOp} from './ops/binary-op';
 import {CpuConcat} from './ops/concat';
 import {CpuConv} from './ops/conv';
 import {CpuDropout} from './ops/dropout';
+import {CpuFlatten} from './ops/flatten';
 import {CpuGather} from './ops/gather';
 import {CpuGemm} from './ops/gemm';
 import {CpuImageScaler} from './ops/image-scaler';
@@ -98,6 +99,8 @@ function createOperator(node: Graph.Node, domain: string, version: number): Oper
       return new CpuConv();
     case 'Dropout':
       return new CpuDropout();
+    case 'Flatten':
+      return new CpuFlatten();
     case 'Gemm':
       return new CpuGemm();
     case 'ImageScaler':
diff --git a/lib/backends/cpu/ops/flatten.ts b/lib/backends/cpu/ops/flatten.ts
new file mode 100644
index 00000000..80f45eb9
--- /dev/null
+++ b/lib/backends/cpu/ops/flatten.ts
@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+import {Flatten} from '../../../ops/flatten';
+import {Tensor} from '../../../tensor';
+import {ShapeUtil} from '../../../util';
+import {CpuInferenceHandler} from '../inference-handler';
+
+export class CpuFlatten extends Flatten {
+  run(inferenceHandler: CpuInferenceHandler, inputs: Tensor[]): Tensor[] {
+    const output = flatten(inputs[0], this.axis);
+    return [output];
+  }
+}
+
+export function flatten(x: Tensor, axis: number): Tensor {
+  const outputDims = ShapeUtil.flattenShape(x.dims, axis);
+  const output = new Tensor(outputDims, x.type);
+
+  const X = x.numberData;
+  const Y = output.numberData;
+
+  Y.set(X);
+
+  return output;
+}
diff --git a/lib/backends/webgl/ops/flatten.ts b/lib/backends/webgl/ops/flatten.ts
new file mode 100644
index 00000000..59777fe8
--- /dev/null
+++ b/lib/backends/webgl/ops/flatten.ts
@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+import {Flatten} from '../../../ops/flatten';
+import {Tensor} from '../../../tensor';
+import {ShapeUtil} from '../../../util';
+import {WebGLInferenceHandler} from '../inference-handler';
+
+import {reshape} from './reshape';
+
+export class WebGLFlatten extends Flatten {
+  run(inferenceHandler: WebGLInferenceHandler, inputs: Tensor[]): Tensor[] {
+    const outputDims = ShapeUtil.flattenShape(inputs[0].dims, this.axis);
+
+    return [reshape(inferenceHandler, inputs[0], outputDims)];
+  }
+}
diff --git a/lib/backends/webgl/session-handler.ts b/lib/backends/webgl/session-handler.ts
index 44081fcb..b4097233 100644
--- a/lib/backends/webgl/session-handler.ts
+++ b/lib/backends/webgl/session-handler.ts
@@ -15,6 +15,7 @@ import * as binaryOps from './ops/binary-op';
 import {WebGLConcat} from './ops/concat';
 import {WebGLConv} from './ops/conv';
 import {WebGLDropout} from './ops/dropout';
+import {WebGLFlatten} from './ops/flatten';
 import {WebGLGather} from './ops/gather';
 import {WebGLGemm} from './ops/gemm';
 import {WebGLImageScaler} from './ops/image-scaler';
@@ -123,6 +124,8 @@ export class WebGLSessionHandler implements SessionHandler {
         return new binaryOps.WebGLBinaryOp(NUMBER_TYPES, binaryOps.glslEqual(), undefined, 'bool');
       case 'Exp':
         return new unaryOps.WebGLUnaryOp(FLOAT_TYPES, unaryOps.glslExp());
+      case 'Flatten':
+        return new WebGLFlatten();
       case 'Floor':
         return new unaryOps.WebGLUnaryOp(FLOAT_TYPES, unaryOps.glslFloor());
       case 'Gather':
diff --git a/lib/ops/flatten.ts b/lib/ops/flatten.ts
new file mode 100644
index 00000000..2a7e8cff
--- /dev/null
+++ b/lib/ops/flatten.ts
@@ -0,0 +1,42 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+import {Attribute} from '../attribute';
+import {InferenceHandler} from '../backend';
+import {Operator} from '../operators';
+import {Tensor} from '../tensor';
+
+export abstract class Flatten implements Operator {
+  abstract run(inferenceHandler: InferenceHandler, inputs: Tensor[]): Tensor[]|Promise<Tensor[]>;
+
+  initialize(attributes: Attribute): void {
+    this.axis = attributes.getInt('axis', 1);  // default axis is 1
+  }
+
+  checkInputs(inputs: Tensor[]): boolean {
+    if (!inputs || inputs.length !== 1) {
+      return false;
+    }
+
+    if (inputs[0].dims.length === 0) {
+      return false;  // scalar tensor is not supported
+    }
+
+    if (this.axis < 0 || this.axis > inputs[0].dims.length) {
+      return false;
+    }
+
+    return this.checkInputTypes(inputs);
+  }
+
+  protected checkInputTypes(inputs: Tensor[]): boolean {
+    // TODO: Support string type
+    if (inputs[0].type === 'string') {
+      return false;
+    }
+
+    return true;
+  }
+
+  protected axis: number;
+}
diff --git a/lib/util.ts b/lib/util.ts
index 07dde8a4..75208c6c 100644
--- a/lib/util.ts
+++ b/lib/util.ts
@@ -667,6 +667,19 @@ export class ShapeUtil {
     return size;
   }
 
+  /**
+   * Determines the shape of output tensor y = flatten(x, axis)
+   * @param dims - shape of input tensor
+   * @param axis - flatten axis
+   */
+  static flattenShape(dims: ReadonlyArray<number>, axis: number): ReadonlyArray<number> {
+    const total = dims.reduce((x, y) => x * y, 1);
+    const right = dims.slice(axis).reduce((x, y) => x * y, 1);
+    const outputDims = [total / right, right];
+
+    return outputDims;
+  }
+
   /**
    * Determines the shape of output tensor y = squeeze(x, axes)
    * @param dims - shape of input tensor
diff --git a/test/unittest-whitelist.jsonc b/test/unittest-whitelist.jsonc
index 19d6884f..25dec7ec 100644
--- a/test/unittest-whitelist.jsonc
+++ b/test/unittest-whitelist.jsonc
@@ -61,6 +61,11 @@
       "test_div",
       "test_dropout_default",
       "test_dropout_random",
+      "test_flatten_axis0",
+      "test_flatten_axis1",
+      "test_flatten_axis2",
+      "test_flatten_axis3",
+      "test_flatten_default_axis",
       "test_gather_0",
       "test_gather_1",
       "test_gemm_broadcast",
@@ -286,6 +291,11 @@
       "test_div",
       "test_dropout_default",
       "test_dropout_random",
+      "test_flatten_axis0",
+      "test_flatten_axis1",
+      "test_flatten_axis2",
+      "test_flatten_axis3",
+      "test_flatten_default_axis",
       "test_gather_0",
       "test_gather_1",
       "test_gemm_nobroadcast",