diff --git a/src/yycc/patch/fopen.cpp b/src/yycc/patch/fopen.cpp
index ede83e4..772e044 100644
--- a/src/yycc/patch/fopen.cpp
+++ b/src/yycc/patch/fopen.cpp
@@ -1,35 +1,38 @@
 #include "fopen.hpp"
 #include "../macro/os_detector.hpp"
-#include "../string/reinterpret.hpp"
 
 #if defined(YYCC_OS_WINDOWS)
-#include "../windows/import_guard_head.hpp"
-#include <Windows.h>
-#include "../windows/import_guard_tail.hpp"
-#endif
-
+// Include header file including MSVCRT specific function.
+#include <wchar.h>
+// Include encoding convertion header
+#include "../encoding/windows.hpp"
+// Define namespace macro
+#define ENC ::yycc::encoding::windows
+#else
+// Include reinterpret header and define namespace macro
+#include "../string/reinterpret.hpp"
 #define REINTERPRET ::yycc::string::reinterpret
+#endif
 
 namespace yycc::patch::fopen {
 
     std::FILE* fopen(const char8_t* u8_filepath, const char8_t* u8_mode) {
-        // TODO: Fix this after finish Windows encoding
-// #if defined(YYCC_OS_WINDOWS)
+#if defined(YYCC_OS_WINDOWS)
+        // convert mode and file path to wchar
+        auto wmode = ENC::to_wchar(u8_mode);
+        auto wpath = ENC::to_wchar(u8_filepath);
 
-//         // convert mode and file path to wchar
-//         std::wstring wmode, wpath;
-//         if (!YYCC::EncodingHelper::UTF8ToWchar(u8_mode, wmode))
-//             return nullptr;
-//         if (!YYCC::EncodingHelper::UTF8ToWchar(u8_filepath, wpath))
-//             return nullptr;
-
-//         // call microsoft specified fopen which support wchar as argument.
-//         return _wfopen(wpath.c_str(), wmode.c_str());
-
-// #else
-//         return std::fopen(REINTERPRET::as_ordinary(u8_filepath), REINTERPRET::as_ordinary(u8_mode));
-// #endif
+        // check convertion success
+        if (wmode.has_value() && wpath.has_value()) {
+            // call microsoft specified fopen which support wchar as argument.
+            return _wfopen(wpath.value().c_str(), wmode.value().c_str());
+        } else {
+            // fail to convert encoding
+            return nullptr;
+        }
+#else
         return std::fopen(REINTERPRET::as_ordinary(u8_filepath), REINTERPRET::as_ordinary(u8_mode));
+#endif
     }
 
-}
+} // namespace yycc::patch::fopen
diff --git a/testbench/CMakeLists.txt b/testbench/CMakeLists.txt
index 8ba0e64..4734267 100644
--- a/testbench/CMakeLists.txt
+++ b/testbench/CMakeLists.txt
@@ -11,6 +11,7 @@ PRIVATE
     yycc/constraint.cpp
     yycc/constraint/builder.cpp
     yycc/patch/ptr_pad.cpp
+    yycc/patch/fopen.cpp
     yycc/string/op.cpp
     yycc/string/reinterpret.cpp
     yycc/num/parse.cpp
diff --git a/testbench/yycc/patch/fopen.cpp b/testbench/yycc/patch/fopen.cpp
new file mode 100644
index 0000000..c294857
--- /dev/null
+++ b/testbench/yycc/patch/fopen.cpp
@@ -0,0 +1,24 @@
+#include <gtest/gtest.h>
+#include <yycc.hpp>
+#include <yycc/patch/fopen.hpp>
+
+#define FOPEN ::yycc::patch::fopen
+
+namespace yycctest::patch::fopen {
+
+    TEST(PatchFopen, Normal) {
+        FILE* handle;
+
+#if defined(YYCC_OS_WINDOWS)
+        // In Windows, we can always visit NUL device.
+        handle = FOPEN::fopen(u8"NUL", u8"wb");
+#else
+        // In other system following UNIX design, we can visit /dev/null device.
+        handle = FOPEN::fopen(u8"/dev/null", u8"wb");
+#endif
+
+        ASSERT_TRUE(handle != nullptr);
+        std::fclose(handle);
+    }
+
+} // namespace yycctest::patch::fopen