This paper presents a new optical configuration for measuring the phase retardation of optical linear birefringent materials. Phase retardation is measured by a heterodyne light source, which is generated by a Zeeman laser. The measurement system has advantages as a simple optical setup, high stability, small size, and portable owing to the configuration and the use of a Zeeman laser. Using the ratio of amplitudes from two measured heterodyne signals, a simple algorithm can obtain the phase retardation directly. Moreover, we can extend dynamic range of the phase retardation measurement to be 0 180 successfully. According to the experimental results, the average absolute error for the phase retardation of λ / 4 -wave plate sample is determined to be only 2.7 %.