To provide a method for designing a phase grating pattern to present deformed illumination and to provide a method for manufacturing a photomask by using the method.

The method for designing a phase grating pattern includes steps of: selecting arbitrary deformed illumination to be provided to the pattern to be transferred onto a wafer in a photolithographic process; determining a unit region where the phase grating is realized; dividing the unit region into many subordinate cells; assigning a desired phase as the initial phase to each subordinate cell; and repeating processes of randomly selecting one of the subordinate cells and varying the phase of the selected cell so as to randomly varying the arrangement of phases assigned to the subordinate cells and to search the arrangement of phases matching with the selected deformed illumination.