Abstract: The effects of irreversible hydrogen trapping on HIC of high strength 42CrMo and 13Ni5CrMoV steel were investigated by thermal desorption spectrometry(TDS)and slow strain rate test(SSRT).SEM and TEM were used to reveal the relationship between microstructure and hydrogen trap type.The results show that due to high carbon content of 42CrMo steel, some -Fe_xC appear during tempering and transformed Fe₃C as the irreversible hydrogen trap exists in the matrix with the activation energy of 68.3kJ/mol.The notch tensile strength decreases distinctly for 42CrMo steel by hydrogen released, and it is proved experimentally that the irreversible is harmful for notched parts.Hydrogen charging has an influence on the strength and plasticity of 42CrMo steel while just on the plasticity of 13Ni5CrMoV steel.The effects of microstructure on the HIC for experimental steels are distinct.Combined the hydrogen trapping and SSRT results, the irreversible hydrogen in the grain boundary and interval of martensite lath transforms to reversible hydrogen in the stress concentration and deteriorate the mechanical properties of 42CrMo steel.