A method for measuring a critical dimension of a mask pattern, including generating a mask pattern using an optically proximity-corrected (OPC) mask design including at least one block; measuring a first critical dimension of a target-region of interest (target-ROI) including neighboring blocks having a same critical dimension (CD), in the mask pattern; determining a group region of interest including the target-ROI and at least one neighboring block adjacent to the target-ROI; measuring second CDs of the neighboring blocks of the group region of interest; and correcting a measuring value of the first CD using a measuring value of the second CDs.