A phase difference between s and p polarizations is produced when a circularly polarized heterodyne light beam is reflected from an air/quartz interface. Under a moderate condition, the phase difference is a function of the extraordinary index ne , the ordinary index no , and the incidence angle. The phase differences that correspond to two different incidence angles are measured. Then two sets of simultaneous equations are derived and two sets of solutions are obtained. Finally, by imposing the condition ne>no for a positive uniaxial crystal, the extraordinary index ne and the ordinary index no of the quartz plate can be estimated.