Failure of nitriding bearing steel is mainly due to fatigue failure, the medium carbon Cr and W nitriding bearing steel tested high-cycle RBF (rotating bending fatigue) test, draw the S-N curve and fatigue up-down graph and analyze the fatigue failure mechanism. Mechanics performance testing, microstructure and FF (fatigue fracture) characteristic observing found that: Its fatigue limit is 729MPa, the microstructure is tempered sorbite. FS (Fatigue Source) are divided into surface defect cracking and the internal non-metallic inclusions cracking. Surface defects are caused of non-metallic inclusions falling off, machining marks, etc; the influence of its buried deepness and size both play an important role in fracture mechanism. Builded a model the effect of inclusions' position and size on the ultimate strength of the medium carbon Cr and W nitriding bearing steel under rotating bending fatigue.