Abstract: The teeth of gear shaft and gear in mill distribution reduction gearbox collapsed in use, the collapse reason was analyzed by means of chemical composition analysis, mechanical properties test and metallographic examination. The results showed that the dry friction was happened between the gear shaft and gear because of lubrication fault, the temperature on gear surface increased and the thermoplastic rheology happened, at last the gear teeth was collapsed and failured. The propose was put forward that lubricating oil supply system fault should be prevented to avoid gear collapse.