Abstract: In this paper, a new theoretical model on hydrogen embrittlement of nanocrystalline metals is proposed, in which hydrogen atoms are assumed to accumulate in the nearest grain boundary ahead of the nanocrack tip; they prevent the emitting of dislocations from crack tip, and thus suppress nanocrack tip blunting as well as induce brittle fracture. The dependence of the critical stress intensity factor on grain size with and without hydrogen in nanocrystalline Ni is clarified and compared. The results show that the ingress of hydrogen into nanocrystalline metals results in a drop in critical stress intensity factor by more than 30% in contrast with the hydrogen free condition, and this hydrogen-induced embrittlement in nanocrystalline metals is especially remarkable with the reduction of grain size.