Abstract: The present invention provides a method of performing optical proximity corrections of a photo mask pattern by using a computer. The photo mask pattern is formed on a photo mask which is used when performing photolithography for forming a predetermined original pattern by exposing a photo-resist layer in a predetermined area of a semiconductor wafer. The photo mask pattern is divided into a plurality of rectangular blocks. Each block can be bright or dark, and a least one side and two corners of the block are shared with another block. Each of shared corners is checked to find corners which may be affected by an optic proximity effect, and those corners are modified so as to prevent them from being affected by the optic proximity effect.