diff --git a/cmake/external/eigen.cmake b/cmake/external/eigen.cmake
index 840b4ecd5a..89a009e39e 100644
--- a/cmake/external/eigen.cmake
+++ b/cmake/external/eigen.cmake
@@ -1,7 +1,7 @@
 include (ExternalProject)
 
 if (onnxruntime_USE_PREINSTALLED_EIGEN)
-    set(eigen_INCLUDE_DIRS ${eigen_SOURCE_PATH})
+    file(TO_CMAKE_PATH ${eigen_SOURCE_PATH} eigen_INCLUDE_DIRS)
     ExternalProject_Add(eigen
         PREFIX eigen
         SOURCE_DIR ${eigen_SOURCE_PATH}