Disclosed is a method of fabricating an assist feature in a photomask, which includes: fabricating a design layout in which main patterns are arranged; setting a critical dimension (a) of assist features to be formed and a spacing (b) between the main pattern and the assist feature; setting a first expanded region extending from the main pattern by (a+b); setting a second expanded region extending from the main pattern by (b); and setting the assist features by removing the second expanded region from the first expanded region.