The present disclosure provides a method of repairing a mask. The method includes inspecting the mask using a mask inspection tool to identify a defect on a circuit pattern of the mask; repairing the defect using a mask repair tool to form a repaired pattern; forming a first group of diffraction images of the repaired pattern and a second group of diffraction images of a reference feature; and validating the mask by comparing the first group of diffraction images with the second group of diffraction images.