Abstract: As an important part of the wind generation, the main gearbox has an great impact on the wind turbine. The causes of teeth breakage in the main gearbox of wind turbine generator was analyzedin this paper. Chemical composition, metallographic organization, mechanical property tests and fracture micro-morphological analysis were carryed out from the broken teeth of the sun wheel and gear ring specimens. It is shown that the sun wheel material material properties were in line with the standard requirements. But the gear ring material was not treated by surface-strengthened, and the metallographic organization was not homogeneous, the appearance of banded structure, resulting in low hardness of the material, strength, impact toughness of the gear ring material. The low strength of the ring material is one of the main reasons for the failure of the gearbox.