This study proposes a method for measuring small displacements. To this purpose, a circularly polarized heterodyne light beam, reflected from a mirror, impinges into a semispherical prism in a radial direction and is reflected at the flat base of the prism passing then through a properly oriented analyzer for interference. The phase difference between - and -polarized light results are sensitive to the impinging angle when it is equal to the internal reflection polarization angle. If the mirror is displaced, then it causes a small variation of the impinging angle and phase changes. The phase difference obtained allows evaluating the mirror displacement through heterodyne interferometry. The feasibility of this method was demonstrated, and the displacement measurement resolution is .