diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5b3f7947d..edc57094b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -23,7 +23,7 @@ IF(MSVC)
 	ADD_DEFINITIONS(-D_HAS_AUTO_PTR_ETC) # for Luabind on C++17
 
 	SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP")
-	ADD_DEFINITIONS( "/W0 /D_CRT_SECURE_NO_WARNINGS /wd4005 /wd4996 /nologo ")
+	ADD_DEFINITIONS( "/W0 /D_CRT_SECURE_NO_WARNINGS /wd4005 /wd4996 /nologo /Os")
 ELSE(MSVC)
 	ADD_DEFINITIONS(-DHAS_UNION_SEMUN)
 ENDIF(MSVC)