Abstract: Gears (gear shafts) are very important structural components on wind turbines. The main failure modes of the gears and their characteristics were introduced. The internal and external causes of gear failure were discussed from the aspects of the force characteristics, material selection characteristics, process characteristics and use characteristics of the gear during normal service. Combined with the actual case analysis, the failure causes of the gears (gear shafts) of the frequently-failed wind turbine generators were summarized. It is found that fatigue fracture and hydrogen induced cracking are the most common failure modes. The metallurgical quality of the materials is the main reason for the failure of the gear (gear shafts) on the domestic wind turbine generator systems, and the preventive measures for the failure of the gear (gear shafts) on the wind turbine generator systems are proposed.