Abstract: Causes for the flaking and tooth breaking of the second shaft gear of a shearer were analyzed by means of macroscopic analysis, chemical composition analysis, metallographic examination and hardness test, combined with mechanical checking and working condition analysis. The results show that there were grinding and quenching layer and re-tempering layer on the tooth surface because of the improper processing technology, which was the main cause for the flaking and tooth breaking of the gear. Some reasonable prevention and improvement measures were also proposed.