A mask includes a substrate, a main pattern, a first assist pattern, and a second assist pattern. The main pattern is disposed on the substrate. The main pattern includes a first pattern and second patterns. Two of the second patterns are disposed at two opposite sides of the first pattern in a first direction. The first assist pattern is disposed on the substrate and disposed in the main pattern. The second assist pattern is disposed on the substrate and disposed outside the main pattern. The first assist pattern disposed in the main pattern may be used to improve the pattern transferring performance in a photolithography process using the mask.