In situ synchrotron small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) were carried out to study the lamellar and crystalline structural evolutions of poly(ethylene terephthalate) (PET) fiber during cold (20 degrees C) and hot (200 degrees C) drawing. The stress-strain curves were divided into two zones, i.e. linearly stress developing zone (zone I) and yield zone around fracture (zone II). In zone I during cold drawing, the two-dimensional (2-D) SAXS pattern transformed from a four-spot pattern to the coexistence of two- and four-spot patterns. The disappearance of lamellar peaks in zone II indicated the destruction of periodic lamellar structures, which was caused by the surface fragmentation of crystalline layers. In contrast, the 2-D SAXS patterns always existed during hot drawing, and the lamellar inclination decreased with strains. The lamellar and fibrillar structures were not destroyed, and crystallization occurred in the interlamellar amorphous layers. Fragmentation of crystalline surface mostly occurred at the end of zone II near fracture process. WAXD data suggested that the crystallinity and crystallite size perpendicular to (010) plane decreased, while the crystallite size perpendicular to (100) plane was rather stable during both cold and hot drawing. (C) 2017 Elsevier Ltd. All rights reserved.