Ultra High Molecular Weight Polyethylene (UHMWPE) fiber has high tensile strength, high wear tolerance and good shock absorbance. And now, it is widely used for engineering applications. However, its long term property, especially fatigue property under dynamic tensile loading, has not been revealed. In addition, there are few studies about oxidation of UHMWPE fiber which causes degradation of mechanical property. In order to undersatand fatigue property and evaluate influence of oxidation and Vitamin-E (α-Tocopherol) on the mechanical properties, we carried out tensile test and fatigue test for virgin specimen, Vitamin-E added specimen and their accelerated aged specimens. For tensile test, the displacement at failure decreased by accelerated aging, but the decrease of displacement at failure was improved by adding Vitamin-E. For fatigue test, virgin specimen was fractured at lower stress than tensile strength influenced by cyclic loading. Furthermore, the fatigue strength was lower for accelerated aging specimen than virgin one.