Abstract: In the report, the failure analysis of gear in reducer of coal mill was performed. The results indicated that the bending fatigue strength of the gear root is not enough, the fracture of the gear tooth belongs to the fatigue fracture caused by the cumulative damage, and there is not only Widmanstatten structure in the gear, there is also a large segregation area, so, there is a large organization stress in the material. The overlap between organization stress and the work stress induce the formation and expansion of crack. The results also showed that the gear surface has not been heat-treated and the surface hardness has not reached the design requirements.