Poly(ethylene terephthalate) (PET) fibers have used widely as industrial materials; for example, tire cords and conveyor belts, etc., In the case of tire cords, fatigue resistance property is important because it is constantly flexed, extended and compressed in a tire. It was well known that long period of gibers affects fatigue resistance property. But how the effect of One structure affects the fatigue resistance property was not known accurately yet. So, in this study the changes of fine structure and the mechanical property of PET tire cord fibers, which have a various long period and is subjected by repeated extension fatigue were investigated to examine fatigue resistance property and fatigue mechanism at the initial fatiguing step. Three kinds of PET fibers with various draw ratio and spinning speed, provided by Toyobo Co., Ltd.. were used in this study. Repeated extension fatigue test of PET tire cord gibers was carried out under a cyclic extension of 2%(with an initial load of 2.5g, different number of fatigue cycle : 105~7$ imes$105) with a frequency of 2-3Hz at room temperature. To investigate fatigue resistance property, molecular weight, density, wide angle X-ray diffraction, mechanical properties, surface of samples and the dynamic viscoelasticity measurements were carried out.