A convenient and accurate technique for measuring the birefringence of liquid crystal(LC) is developed. The birefringence of LC was obtained with only one experimental run by using phase modulation technique, which is an expansion of PMFB(Phase Modulated Flow Birefringence)technique developed elsewhere[1]. The basic principle of the method is based on the determination of the phase differences which occur when a phase modulated monochromatic polarized light propagates through a medium with an anisotropic refractive index. Birefringence measurements with red light(wavelength : 0.6328㎛) for commercial liquid crystal material(Merck, ZNL 3201-000) are reported.