Abstract: A gearbox in the 1.5MW wind turbine failed in the service, and the tooth of the small gear on the medium-speed shaft cracked seriously. Macro-observation, micro-observation, fracture observation and some other physical and chemical tests were adopted to analyze the reason for the failure of the cracked gear. Finally, the cause for the failure of the gear was analyzed systematically. The results show that the failure mode of the small gear is fatigue fracture, according to the beach marks observed on the fracture surface. The fatigue crack started at the bottom of the pits caused by the contact fatigue, which should be attributed to the unbalanced load.