Abstract: This article focused on the cracking of 42CrMo fuel injector body in a marine diesel engine. The causes of cracking were studied by means of macroscopic and microscopic observation of fracture surface, chemical composition testing, hardness testing, and metallographic examination. The results showed that the failure form of the fuel injector body was fatigue cracking. The machining quality of the R position of oil hole was poor, induction hardening resulted in higher hardness and coarse microstructure at this position. The R position generated stress concentration during use, leading to fatigue cracking under alternating load.