The objective of this study is to develop a systematic method to determine molecular weight and its distribution of flexible chain polymer by birefringence experiments. Using the random walk model, birefringence Δn and orientation angle χ have been optically obtained as functions of molecular weights. To confirm the theory, polystyrene solutions with different molecular weights dissolved in polychlorinated biphenyl were experimented by the phase-modulated flow birefringence (PMFB) method. Birefringence of polystyrene solutions is proportional to (∑ciMi(1.6))γ(2), and cot(2χ) to (∑ciMi(1.6)/B1∑ciMi(-0.2))γ. The experimental results agreed well with the theoretical predictions proposed in this study.