In order to investigate the effect of molecular on fatigue characteristics in ultra high molecular weight polyethylene (UHMWPE), tension-tension fatigue tests with notched specimen were carried out in the present study. The effects of frequency and stress ratio were also investigated and fractography was discussed. Fatigue strength is not increased with increasing molecular weight. Samples made from particles which have smallest diameter or narrow particle size distribution show improved fatigue strength. Fatigue strength might be influenced by its high degree of crystallinity in spite of decreased tie molecule density. There is almost no effect of frequency on number of cycles to failure. However, the higher the frequency, the higher the crack tip temperature. Heat and strain rate effects must be considered in high polymer materials. In high stress ratio, S-N curves shift to high cycle side. The fatigue life of UHMWPE is influenced by both stress amplitude and mean stress.