Methods of fabricating a photo mask are provided. The method includes collecting sample data, setting a preliminary mask layout, performing an optical proximity correction using the sample data and a preliminary mask layout to obtain an optimized preliminary mask layout, verifying the optimized preliminary mask layout to obtain a final mask layout, and fabricating the photo mask using the final mask layout. Verification of the optimized preliminary mask layout includes operating a verification simulator using the sample data and the optimized preliminary mask layout as input data to obtain verification image data. The verification image data includes a plurality of contours of a pattern at different vertical positions.