Hydrogen (H) embrittlement of metals is a common phenomenon but the exact atomic mechanisms responsible for H-induced plasticity process and ultimate failure are not clarified. In this work, the impacts of H on tensile deformation behaviour of different types of twist grain boundaries (TGBs) have been systematically studied by molecular dynamics (MD) simulations. Different deformation mechanisms are reported, depending on grain boundary types and bulk H concentrations, including easier dislocation nucleation due to the presence of H, H-enhanced dislocation dissociation, and H-induced slip planarity. The simulations indicate that the interactions between H-enhanced dislocation plasticity and TGBs play a crucial role in the ultimate fracture path. The decohesion of the TGBs is considerably promoted by the presence of H under conditions where dislocation accommodation and emission process on the TGBs causes the changes of grain boundary structures and local stress state. Our results advance a mechanistic understanding for experimentally-observed H embrittlement and provide a viable path to engineering microstructure with high resistance to H embrittlement in new materials.