Abstract: Interferogram processing is one of main techniques in optical interferometry metrology. Modern interferometry for thin-film thickness has the advantages of non-contact, high accuracy and great field of view, etc. Taking advantage of Twyman-Green interferometer, the relationship between interferogram and measured object parameter can be determined by mathematical model, thus the thickness of measured object is measured automatically. A new algorism of phase unwrapping for measuring the material object is proposed based on the FFT method, an algorism of diamond phase unwrapping. This algorism identifies a seed point, seed points spread to four points nearby the fist seed point, the four points will serve as the second group of seed points, these seed points will spread to four points nearby the second group of seed points in turn and pass through all of the effective information points by a diamond path. Seed points will eventually bring about the phase unwrapping in the entire image. In this paper, the thin-film thickness is determined by the method, setting the results of this method against results from ZYGO, the PV error and RMS maximum error are 0.0364 and 0.002 respectively. Although single interferogram was processed in this study, a phase distribution with high accuracy was achieved.