The evolution Of Superstructure and its relationship with the phase transition during uniaxial deformation of poly(ethylene terephthalate) (PET) at temperatures (90 and 100 degrees C) above its -lass transition temperature were investigated by in-situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). It appears that deformation at lower temperatures enhances the metastability of niesophase but narrows the strain window for phase transition. Very similar superstructure evolution pathways were observed at both temperatures. In zone I (the plastic deformation zone), WAXD did not show any crystal diffraction peak; however, SAXS exhibited an equatorial streak at the later stage, indicating the formation of a microfibrillar structure. Strain hardening took place in zone II, which could be categorized in two substages. In zone II-a, SAXS showed an X-shaped pattern that coincided with the appearance of crystal diffraction peaks in WAXD. The initial X-shaped patterns possessed strong intensity near the beam stop; the later patterns exhibited a scattering maximum that shifted toward larger angles with increasing strain. Results indicated the formation of a tilted lamellar structure within the microfibrils in conjunction with lamellar insertion. In zone II-b, oval spots appeared at the edges of the X-shaped pattern, which essentially became four-point. In this stage, the crystallinity still increased linearly with strain, but the invariant gradually reached in asymptotic value, indicating that lamellar insertion took place at a slower rate. In the final strain-hardening zone (III), the load became linear with strain even though crystallization was reduced. The equatorial long period was found to decrease drastically, suggesting that some microfibfils were split. In addition, a two-point pattern appeared near the central streak, corresponding to a periodic structure with long period of 100 nm. The formation of such a large layered structure and the in icrofibrillar splitting can be attributed to structural defects such as kinks in microfibrils.