Abstract: Frequent preliminary fracture of six speed gears of transmission have been investigated by means of analysis on fracture feature, microstructure hardness and so on. The results showed that the fracture model was low stress and high cycle fatigue fracture. It was indicated by crack propagation process that the insufficient design strength of gear spoke was the primary cause of the kind of gear fatigue fracture in the primary stage. Accordingly, such problem would be resolved by bending stress reduction of gear root, which could be achieved through the design modification of gear tooth shape and oilhole.