Abstract: In this paper the effects of diffusible hydrogen content and cooling rate (t_8/3) on hydrogen-induced cracking (HIC) of steel 20MnNiMo have been investigated by implant teat,and the fractography of implant is observed by scanning electron microscope (SEM). The experimental results show that HIC of steel 20MnNiMo is relative with diffusible hydrogen content strongly. At low hydrogen level, high restrain stress is needed to nucleate HIC; but at high hydrogen level,only small restrain stress can cause HIC occurrence. At the same hydrogen content level,the better the HAZ toughness, the higher resistance to HIC.