Abstract: By means of macro and micro fracture morphology observation, chemical composition analysis, microstructure observation and hardness test, the failure causes of the boost gear of a large capacity wind power generator were investigated systemically. The results show that densely distributed pits and micro-voids existed on the external surface of the gear, which were the crack origins, caused the mirco cracks propagation under the combined action of severe impact and bending load, and finally resulted in the fatigue fracture.