Orientation-induced crystallization of PET fibers was studied by the in-situ wide-angle X-ray diffraction (WAXD) utilizing synchrotron radiation source combined with thermomechanical analysis. The noncrystalline as-spun fiber spun was heat-treated at 150, 165, 180 and 195 degrees C for 0.1 s under constrained condition. The heat-treatment allowed the fibers to have various amount of isotropic amorphous (IA), oriented noncrystalline (ON), and crystalline (Cr) phase. The structure evolution accompanying the crystallization of the fibers was then examined upon elevating temperature while the fiber length was held constant. The X-ray results clearly showed that the crystallization takes place first by ON phase (extended-chain crystallization) and then followed by the crystallization of IA phase (folded-chain crystallization). The on-set of extended-chain crystallization was dependent on the amount and degree of orientation of ON phase in the fiber that was derived from the various heat-treatment temperatures. It is also noted that the IA phase transforms into not only the CR phase but also the ON phase. The crystallization on the surface of preformed extended-chain crystals appeared to induce the spontaneous orientation of the chains. The thermomechanical data indicated that a stress emerges rapidly on fiber above glass transition temperature (T-g), which is associated with the entropic relaxation of the ON phase. The stress emerged on fiber then dropped drastically as the temperatures of fibers reached the temperatures of extended-chain crystallization, indicating that the stress drop is closely related with the extended-chain crystallization. The fibers heat-treated at the highest temperature showed the highest initial crystallinity but showed the slowest crystallization rate, resulting in the lowest final crystallinity among the fibers. (C) 2008 Elsevier Ltd. All rights reserved.