Isothermal crystallization from the glassy state of oriented isotactic polystyrene (iPS) was studied using in situ Fourier transform infrared (FTIR) spectroscopy and in situ wide-angle X-ray diffraction (WAXD) studies. The oriented amorphous films of iPS were prepared by rolling the amorphous iPS film to a draw ratio of 3 or 4. In situ FTIR was used to investigate the ordering process of polymer chains prior to crystallization by measuring the change in the dichroic ratio with time, while in situ WAXD studies were used to investigate the development of the crystalline structure. The studies showed that the orientation process and the conformation change preceded crystallization. This observation suggests that polystyrene chains undergo an ordering process during the induction period of crystallization. The degree of orientation markedly increases with time in the induction period, suggesting that heat treatment of oriented amorphous materials under constraint provides a useful method for processing highly oriented materials.