A mask assembly including a first mask and a second mask is provided. The first mask includes a plurality of first main features parallel to each other, a plurality of first sub-resolution assistant features (SRAFs) and a plurality of second SRAFs. The second SRAFs are separately disposed at one side of the first main features. The first SRAFs are separately disposed between the first main features and the second SRAFs. An extension direction of the first main features is parallel to an extension direction of the second SRAFs. The second mask includes a plurality of second main features parallel to each other. When the first mask and the second mask are placed at a predetermined position above a negative-type development photoresist layer for performing exposure respectively, the second main features intersect with the first main features and the second main features overlap with the first SRAFs.