Abstract: The macroscopic and microscopic fracture morphology, metallographic structure, mechanical properties, chemical composition, and hydrogen content of fractured cylinder head bolts were analyzed. By comparing the surface and core hardness test results of the bolts, the analysis indicated that the failure mode of the cylinder head bolts was hydrogen embrittlement. The root cause of the hydrogen embrittlement in the bolts was surfac carburization, which led to excessively high hardness and induced hydrogen embrittlement fracture.