The previous study has shown that the changes in fine structure on cyclic extension of PET tire cord mainly occur in the amorphous region and is due to the two effects occurring at the same time; i.e. strain-hardening effect and increase of defects caused by chain scission. In this study, the changes in fine structure and physical properties of PET tire cord with different aging time and fatigue cycles are investigated. In order to investigate the changes in fine structure with aging time and fatigue cycles, we carried out DSC, TMA, wide angle x-ray diffraction, density, SEM, and rheovibron measurements. Fine structure and physical properties at 10,000 fatigue cycles changed more profoundly than those at the 300,000 fatigue cycles. So, the irreversible effect (chain scission effect) seems to be stronger than the reversible effect (strain-hardening effect) with increasing fatigue cycles. Fatigue failure accompanies a distinctive fracture morphology, clearly different from the tensile fracture morphology. As for the fatigue fracture of the PET and nylon tire cords, fracture end of the PET tire cord is longer than that of nylon tire cord.