diff --git a/diff_encoder_unittest.cc b/diff_encoder_unittest.cc
new file mode 100644
index 0000000..3bbce87
--- /dev/null
+++ b/diff_encoder_unittest.cc
@@ -0,0 +1,74 @@
+// Copyright 2017 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "bsdiff/diff_encoder.h"
+
+#include <vector>
+
+#include <gtest/gtest.h>
+
+#include "bsdiff/fake_patch_writer.h"
+#include "bsdiff/test_utils.h"
+
+namespace {
+
+// Generated with:
+// echo 'Hello World' | hexdump -v -e '"    " 12/1 "0x%02x, " "\n"'
+const uint8_t kHelloWorld[] = {
+    0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x57, 0x6f, 0x72, 0x6c, 0x64, 0x0a,
+};
+
+}  // namespace
+
+namespace bsdiff {
+
+class DiffEncoderTest : public testing::Test {
+ protected:
+  void SetUp() { EXPECT_TRUE(fake_patch_.Init()); }
+
+  FakePatchWriter fake_patch_;
+  std::unique_ptr<DiffEncoder> diff_encoder_;
+};
+
+TEST_F(DiffEncoderTest, CreateEmptyPatchTest) {
+  diff_encoder_.reset(new DiffEncoder(&fake_patch_, nullptr, 0, nullptr, 0));
+  EXPECT_TRUE(diff_encoder_->Close());
+
+  // Both diff and extra stream must be empty stream, and not control entries.
+  EXPECT_EQ(0U, fake_patch_.entries().size());
+  EXPECT_TRUE(fake_patch_.diff_stream().empty());
+  EXPECT_TRUE(fake_patch_.extra_stream().empty());
+}
+
+TEST_F(DiffEncoderTest, AllInExtraStreamTest) {
+  diff_encoder_.reset(new DiffEncoder(&fake_patch_, nullptr, 0, kHelloWorld,
+                                      sizeof(kHelloWorld)));
+
+  // Write to the extra stream in two parts: first 5 bytes, then the rest.
+  EXPECT_TRUE(diff_encoder_->AddControlEntry(ControlEntry(0, 5, 0)));
+  EXPECT_TRUE(diff_encoder_->AddControlEntry(
+      ControlEntry(0, sizeof(kHelloWorld) - 5, 0)));
+  EXPECT_TRUE(diff_encoder_->Close());
+
+  EXPECT_EQ(2U, fake_patch_.entries().size());
+  EXPECT_TRUE(fake_patch_.diff_stream().empty());
+  std::vector<uint8_t> hello_world(kHelloWorld,
+                                   kHelloWorld + sizeof(kHelloWorld));
+  EXPECT_EQ(hello_world, fake_patch_.extra_stream());
+}
+
+TEST_F(DiffEncoderTest, AllInDiffStreamTest) {
+  diff_encoder_.reset(new DiffEncoder(&fake_patch_, kHelloWorld,
+                                      sizeof(kHelloWorld), kHelloWorld,
+                                      sizeof(kHelloWorld)));
+  EXPECT_TRUE(
+      diff_encoder_->AddControlEntry(ControlEntry(sizeof(kHelloWorld), 0, 0)));
+  EXPECT_TRUE(diff_encoder_->Close());
+
+  EXPECT_EQ(std::vector<uint8_t>(sizeof(kHelloWorld), 0),
+            fake_patch_.diff_stream());
+  EXPECT_TRUE(fake_patch_.extra_stream().empty());
+}
+
+}  // namespace bsdiff