In a method of manufacturing a photomask, a layout of a target pattern to be formed in the photomask may be designed. A modeling of a displacement error of the target pattern may be performed and a displacement error correction map of the target pattern may be generated based on the modeling of the displacement error of the target pattern. An exposure process may be performed on a blank mask in reflection of the displacement error correction map of the target pattern. A developing process and an etching process may be performed on the blank mask to form the photomask.