Abstract: The planetary gear of a wind turbine had broken teeth after about 2 years of operation.Using chemical composition analysis,macroscopic observation,fracture analysis,metallographic inspection,scanning electron microscope and energy spectrum analysis,the reasons for broken teeth of the gear were analyzed.The results showed that there were a large number of aggregated inclusions in the raw material of the planetary gear.During the service process of the gear,stress concentration was easy to occur near the inclusions,so that cracks were initiated there,and the surface burn of the gear accelerated the crack propagation,which eventually led to fatigue fracture of the gear.