Time-resolved synchrotron radiation small-angle X-ray scattering(SAXS)and wide-angle X-ray diffraction(WAXD)measurements were performed to study structure evolution of poly(ethylene terephthalate)(PET)industrial yarns during tensile deformation at 125 oC.We found that both crystalline and amorphous phase were involved during the tensile process.WAXD data indicated that the crystallinity of the PET yarns decreased with increasing strain up to the fracture.The estimated crystallite size associated with the(010)reflection remained unchanged at strains under 15%and decreased quickly at strains from 15%to 32%(breaking point).The crystallite size associated with(-110)reflection decreased,while the crystallite size of(100)reflection nearly remained unchanged during the whole tensile process.SAXS data showed that the long period,crystalline and amorphous thickness increased with the increasing strain during tensile deformation.The enlargement of long period indicated that the lamellar structures of the yarn became loosely packed during the tensile deformation.