Abstract: Outer ring of a pitch bearing in a wind turbine generator fractured when used for a period of time. The reason for the fracture was analyzed by methods such as macro-inspection, chemical composition analysis, mechanical property test, fracture morphology and microstructure observation. The results show that the bearing outer ring fractured in a fatigue mode. Pits on inner surface of the bolt hole were generated by corrosive mediums, resulting in stress concentration points. By a relatively large external force, the cracks originated at the pits where existing obvious stress concentration, and consequently led to fatigue fracture.