The phase-modulated flow birefringence (PMFB) method is widely accepted as one of the most sensitive and accurate techniques suitable for experimental tests on the molecular theory of polymer solutions. The objective of this study is to develop a systematic method to determine molecular weight and distribution of rigid-rod polymers by the PMFB technique. Using molecular theory for rigid polymers, birefringence Delta n and orientation angle chi have been expressed as a function of molecular weight and distribution. Delta n has been shown to be proportional to Sigma c(i)M(i)(4), and cot 2 chi turned out to have a linear relationship with Sigma c(i)M(i)(7)/Sigma c(i)M(i)(4). From the experimental results for PBLG solutions, birefringence and orientation angle data were in some degree matched with the theory presented.