In the field of optical measurement, phase always represents the physical quantity to be measured. Thus, phase retrieval from a fringe pattern is a key step for quantitative measurement and evaluation. Much research work has been conducted to develop phase evaluation methods such as fringe tracking and fringe skeletons in earlier, and the more precise methods of phase-shifting and Fourier transform more recently. For phase evaluation, the phase-shifting method requires three or more phase-shifted speckle patterns at each deformed stage; thus, it is not suitable for measurement of continuous deformation. The Fourier transform, on the other hand, requires a high-frequency carrier for phase separation in the spectral domain, which places an additional requirement on experimental arrangement. Thus, it would be desirable to develop a convenient method that can retrieve the modulated phase from a single fringe pattern. We propose an approach that utilizes the phase-clustering property to extract phase information from a single interference specklegram. To explore the ability and limitation for the proposed technique, typical shearographic fringe patterns are used for phase evaluation. Results obtained are similar to those from the standard four-step phase-shifting method. Nonrepeatable continuous movement is also measured by the proposed method, and the results confirm the robustness and accuracy of the clustering method.