Abstract: By means of chemical composition analysis,mechanical property testing,scanning electron microscopy (SEM) and electron diffraction spectrometer(EDS) etc.,the fracture reason of some 30CrMnSiA bolts used for load-carrying vehicle was analyzed.The results show that the fracture of the bolts was delayed brittle failure induced by hydrogen.It was proposed that mechanical zinc plating could not completely avoid hydrogen embrittlement.When the material is with had high inbeing hydrogen content and high tensile strength,the risking of hydrogen embrittlement is high if the machine part was made by mechanical zinc plating because of lack of the process to remove hydrogen.