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 profile of diamond grid disk. To eliminate erroneous bright or black spots caused by the diamond grids, a new approach is proposed by automatically selecting a proper threshold value from the differentiated image resulting from the addition of four phase-shifted images. According to the largest size of erroneous spot, the size of a structuring element is determined for morphology filtering. Thereafter the phase can be calculated and unwrapped correctly. Test of the method on a diamond grid disk is demonstrated and discussed.