The stress relaxation and simultaneous birefringence variation were measured for bisphenol A polycarbonate over the glassy to the rubbery plateau regions. The measurements were performed at various temperatures of 142–156 °C over the time range of 0.4 to 2000 s. Results of relaxation measurements were consistent with those of dynamic measurements with respect to the birefringence as well as the stress in the framework of linear viscoelasticity. A modified stress‐optical rule was applied to the results of relaxation measurements. This rule was earlier proposed to replace the stress‐optical rule which was suitable only in the rubbery and the terminal flow regions. The relaxation modulus,E(t), was separated into two component functions,ER(t) andEG(t). In constructing master curves ofER(t) andEG(t) with the method of reduced variables, their shift factors were found to have different temperature dependence. This result can account for the break down of time‐temperature superposition principle forE(t) reported by several investigators. The present modified stress‐optical rule was more convenient in several ways compared with the similar modifications proposed earlier by Priss et al. and Read.