Seven experimental poly(ethylene terephthalate) (PET) fibers have been spun and then drawn at different process conditions, i.e., spinning speed and draw ratio, in such a way that the fibers possess different long periods but retain the crystal structure. Wide angle X-ray diffraction, small angle X-ray scattering, and loss modulus measurements have been used to characterize the fine structure of the fibers. The influence of fine structure on the extensional fatigue behavior of the PET fibers has studied by subjecting them to 120~l8Orev/min repeating deformation at 10$^4$~ 10 5 cycles. In order to detect the molecular motion of PET concerned with the extensional fatigue, thermoluminescence(TL) experiments were performed at high temperature region(above room temperature). The high temperature TL intensity decreased with l0$^4$ cycle extension but increased with l05 cyclic extenxion. The extent of changein TL intensity was found to be a function of long period and loss modulus.