Abstract: An optical aperture synthesis telescope such as the Multiple-Mirror Telescope will suffer from phase errors unless the segments are aligned to within a small fraction of a wavelength. As an optical misalignment sensing technique, phase diversity can estimate the pupil phase from pairs of simultaneously recorded focused and defocused images, then the misalignment error is obtained. The image-forming system have been simulated by computer, and the unknown wavefront have been estimated by using the method of finite differences. Simulation results show phase-diversity method can accurately retrieve the wavefront, and sense misalignment error.