In a method of manufacturing a photomask, a layout of a circuit mask pattern in a mask region corresponding to a chip region of a substrate is designed. A layout of a monitoring mask pattern representing a critical dimension (CD) of the circuit mask pattern in the mask region is designed. The monitoring mask pattern includes a mask-critical dimension uniformity (CDU) detection pattern configured to detect CDU in mask and a wafer-CDU detection pattern configured to detect CDU in wafer. A first optical proximity correction (OPC) is performed on the mask-CDU detection pattern. A second optical proximity correction is performed on the wafer-CDU detection pattern. A photomask having the circuit mask pattern and the monitoring mask pattern is formed.