Abstract: The microstructure,morphology of cracks,composition and mechanical properties of cracked 42CrMo long shaft after induction quenching were characterized.The causes of the cracks were analyzed.The results show that the longitudinal cracks are found at the hardened layer of the larger diameter region.The grains are abnormally coarse and parts of the grain boundaries are fused compared with those of the center region.Cracks with oxides extends interannually.Therefore,the reason is due to the inappropriate parameters of the induction hardening process for the larger diameter part,which results in the over high quenching temperature,over burned grains and fused grain boundaries and thereafter cracks in the hardened layer.The thicker hardened layer and lower hardness of the larger diameter part compared with those of the smaller diameter part also testify this judgment.