Abstract: The 42CrMo alloy structural steel studs of a diesel injector appeared early fracture. The fracture causes were analyzed by means of macroscopic analysis, chemical composition analysis, hardness test, metallographic examination, process analysis, re-tempering test and so on. The results show that the tempering temperature was too low, the microstructure transformation was insufficient, which results that the hardness and tensile strength of the studs were too high and hydrogen embrittlement sensitirity increased. At the same time, the poor surface quality of the threads aggravated the stress concentration. The combined action resulted in hydrogen induced delayed fracture of the studs.