The objective of this study is to confirm Doi's molecular theory of the solution of rigid rodlike polymers by analyzing experimentally the transient flow birefringence of collagen solutions. Phase-modulated flow birefringence measurements were performed in the transient flow condition to obtain birefringence and orientation angle of sample simultaneously in a single experimental run. Following the cessation of simple shear flow, relaxation of birefringences was recorded to show two distinct relaxation times due to the molecular weight distribution of calf skin collagen, which turns out to be bimodal. initial rapid drop corresponds to the relaxation of a short chain (monomer) as well as a long chains (dimer), but slow decay follows due to relaxation of long chains only. For a bimodal system consisting of monomer and dimer, such as calf skin collagen. not only the mole fraction hut also the rotational diffusivity can be determined experimentally from thc relaxation of birefringence. Rotational diffusivity decreases rapidly with concentration increase around C=0.94 mg/ml, as expected. But the semidilute regime starts at a rather large value of cL3 owing to the flexibility of polymers chains, which can be resolved by using the effective length proposed by Doi.