A method of optical proximity correction (OPC) includes the following steps. First, a layout pattern is provided to a computer system. Subsequently, the layout pattern is classified into a first sub-layout pattern and a second sub-layout pattern. Then, an OPC calculation based on a first OPC model is performed on the first sub-layout pattern so as to form a corrected first sub-layout pattern and an OPC calculation based on a second OPC model is performed on the second sub-layout pattern so as to form a corrected second sub-layout pattern. Afterward, the corrected first sub-layout pattern and the corrected second sub-layout pattern are output from the computer system into a photomask.