Diamond grid disk dresser is frequently employed to remove the accumulated debris lest the polishing surface glazes. The surface warpage of diamond grid disk must be small enough to assure the flatness of polished wafers during chemical mechanical planarization process. In this study, phase-shifted shadow moiré method was employed to measure the surface warpage of diamond grid disks. To eliminate erroneous bright or black spots caused by the diamond grids, a novel method is proposed by selecting proper threshold values from the addition of four phase-shifted images, and from the grey-level difference between the addition of phases 0 and π images and the addition of phases π/2 and 3π/2 images. Test of the proposed method on real specimens show that the erroneous bright and black spots can be effectively identified and patched. Thereafter the phase can be unwrapped successfully to obtain the surface profile and thus the warpage of specimens.