The relationship between the intrinsic birefringence and the composition ratio and between the photoelastic coefficient and the composition ratio in poly(methyl methacrylate (MMA)/styrene (St)) was investigated. A linear relationship was observed between the orientational birefringence and the composition ratio but not between the photoelastic coefficient and the composition ratio. The influence of the compositional distribution on the birefringence was investigated, and the results suggested that the compositional distribution of poly(MMA/St) minimally affects birefringence. We also analyzed the mechanism by which birefringence is generated in poly(MMA/St) by focusing on changes in St conformation. The results of this analysis suggested that the conformational changes in St units are different for orientational and photoelastic birefringence generation. We analyzed the generation mechanism of birefringence in poly(methyl methacrylate (MMA)/styrene (St)) through measurement of infrared (IR) dichroism. Through IR measurement, the conformational changes in the St unit were suggested and found to differ between orientational and photoelastic birefringence generation. We thought that, during photoelastic birefringence generation, the α-CH3 group of the MMA unit hinders the rotation of the benzene ring of St units that are adjacent to MMA units. The influence of compositional distribution on birefringence was also investigated and the results suggested that the compositional distribution of poly(MMA/St) minimally affects the birefringence.