Abstract: Driven gear cracking was found in repair. In order to find out the reason of driven gear cracking,the methods of macro-examination,chemical composition analysis,microstructure examination and micro-hardness testing were carried out to analyze the cracked driven gear. The results showed that tempered soft zone in harden zone case produced in process of discontinuous medium frequency induction hardening which has microstructure of tempered sorbite and low hardness and low fatigue limit were major reason to cause driven gear cracking. According to analytic result,we put forward the suggestion.