Abstract: A tooth of reel asteroid gear fractured abnormally during operation. In order to find out how it occurred，fracture of the gear were analysed by macroscopical observation and microscopical examination. The results showed that the gear failure was multiple origins fatigue fracture，and cracks were also found at the dedendum of most other teeth of the gear. There were bainite and ferrite in gear base material，which showed the quenching cooling rate was too slow. Surface quenching was uneven. All of these problems lead to the decrease of the gear comprehensive mechanical properties. Some fatigue crack origins occured at the dedendum of teeth under the circulation work stress and stress concentration and the cracks expanded， finally caused the gear fracture.

Key words: fatigue fracture, quench, bainite, hardening zone