A method of correcting an optical parameter in a photomask is provided. The method includes providing a photomask, exposing the photomask, detecting an aerial image to estimate the photomask, and irradiating gas cluster ion beams to the photomask based on an estimation result to correct the optical parameter in the photomask in relation to the aerial image. The gas cluster ion beams may be irradiated to a front surface of the photomask on which a mask pattern is formed or a rear surface of the photomask on which the mask pattern is not formed.