Abstract Isothermal crystallization from the glassy state of oriented poly(ethylene terephthalate) (PET) was studied using in situ Fourier transform infrared (FTIR) spectroscopy, in situ wide-angle X-ray diffraction (WAXD) studies, and small angle X-ray scattering (SAXS) studies. The oriented amorphous films of PET were prepared by rolling melt-quenched films to a draw ratio of 2. In situ FTIR was used to investigate the ordering process of polymer chains prior to the crystallization by measuring the change in dichroic ratio with time while in situ WAXD was used to investigate the development of crystalline structure. The SAXS studies of the heat-treated PET showed that the density fluctuation was formed in the induction period. The density fluctuation is nearly isotropic in the early stage of the structural organization, but gradually orients to the stress direction until the onset of crystal growth. The long period decreases as crystallization proceeds, resulting in the formation of the regular long period of lamellar stacks. The structural organization during the isothermal crystallization of oriented PET consists of three stages. The first stage is the thermodynamic relaxation, occurring when the samples are heated above its T g. The second stage is the self-organization process of the oriented amorphous structure, in which the degree of orientation increases with time from nearly isotropic state and the gauche conformation is transformed into the trans conformation. The crystalline reflections begin to appear in the third stage only after the orientation process is completed. These observations suggest that PET chains undergo ordering process in the induction period of crystallization.