A new optical automatic focusing technique based on laser interference principle is proposed. The object wave passing through the imaging lens interferes with the reference wave. Information out of focus can be obtained from the interferograms, specifically from the shifting of interference fringes. The focusing movement of the optical system is driven by an electronic system consisting of a CCD detector, microcomputer and stepping motor. The meachanism of automatic focusing is readily to be interpreted. The novel system has been applied to the instrument for testing the resolving capability of imaging materials. The focusing accuracy can reach 0. 3 μm in the system.