A method of manufacturing a semiconductor device includes dividing a design pattern layout into a repetitive pattern part and a non-repetitive pattern part, obtaining an optical proximity correction (OPC) bias from an extracted portion, the extracted portion being a partial portion of the repetitive pattern part, applying the OPC bias obtained from the extracted portion equally to the extracted portion and other portions of the repetitive pattern part so as to form a first corrected layout in which corrected layouts of the other portions are the same as that of the extracted portion, and forming a photomask in all portions of the repetitive pattern part according to the first corrected layout.