Abstract: Some pitch bearing bolts broke after using for six months in a wind power company. By means of metallographic examination, chemical composition analysis, hardness testing and fracture analysis, the fracture reason of the bolts were analyzed. The results showed that the cracks originated at the R corner between the nut and screw, where was the stress concentration position. After the bolts loosed, the gasket impacted the R corner of bolt and damaged the surface of screw under the alternating load. When the damage reached a certain level, and there was the decarburization phenomenon on the surface of screw, resulted in the initiation of surface crack, and the cracks extended under the alternating load, at last the fatigue fracture occuredin bolts.