Although　hydrogen　embrittlement　(HE)　behavior　has　been　extensively　studied　in　bulk　materials,　little　is　known　about　H-related　deformation　and　the　fracture　of　nanograined　materials.　In　this　study,　H　segregation　and　HE　mechanisms　of　nanograined　Fe　with　different　grain　sizes　are　unveiled,　following　the　employment　of　classical　molecular　dynamics　simulations.　The　H　segregation　ratio　increased,　but　the　local　H　concentration　at　the　grain　boundaries　(GBs)　decreased　with　decreases　in　the　grain　size　at　a　given　bulk　H　concentration.　The　results　demonstrate　that　H　atoms　increased　the　yield　stress　of　nanograined　models　irrespective　of　the　grain　size.　Furthermore,　it　is　revealed　that　brittle　fractures　were　inhibited,　and　the　resistance　to　HE　increased　as　the　grain　size　decreased,　due　to　the　fact　that　the　small-grain　models　had　a　lower　local　H　concentration　at　the　GBs　and　an　enhanced　GB-mediated　intergranular　deformation.　These　results　are　a　clear　indication　of　the　utility　of　grain　refinement　to　resist　H-induced　brittle　failure.