A method of manufacturing a photomask is described. The graphic data of the photomask are provided, and than an optical proximity correction is performed to the graphic data. A process rule check is then performed to the graphic data with the optical proximity correction. When at least one failed pattern not passing the process rule check is found in the graphic data, a repair procedure is performed only to the failed pattern so that the failed pattern can pass the process rule check. The patterns of the photomask are then formed according to the corrected and repaired graphic data.