A method for fabricating a phase shift mask includes preparing a transmissive substrate on which a first mask region and a second mask region surrounding the first mask region are defined. In the first mask region, main patterns are formed having a first pitch in a first direction and a second direction perpendicular to the first direction. Each of the main patterns has a first area. In at least one row, assist patterns are formed at the first pitch to surround the main patterns. Each of the assist patterns has a second area less than the first area. In the second mask region, dummy patterns are formed in a plurality of rows. The dummy patterns surround the assist patterns at the first pitch. Each of the dummy patterns has a third area greater than the first area.