This paper introduces a dual-path Mach-Zehnder interferometer for determining refractive index and geometrical thickness of an optical plate. In this interferometer, two parallel incident beams are separated into two interference pairs which are then recombined to generate two interference signals. An optical plate is placed on the path of one wave of an interference pair, so the phase difference of the interference signals is a function of the plate, and the interferometer is thus able to determine the index and thickness of the plate. A setup constructed to realize the proposed interferometer is described. And the experimental results of applying this setup agree the validity of the interferometer.