Small　gas　molecules　acting　as　good　capping　agents　play　important　roles　in　controlling　the　morphologies　and　surface　structures　of　metal　nanocrystals.　In　the　present　work,　the　thermodynamic　and　kinetic　roles　of　H-2　molecules　in　the　morphology　of　Co　nanocrystals　were　systematically　studied　by　density　functional　theory　(DFT).　The　Gibbs　surface　free　energies　of　Co(100),　Co(110),　and　Co(111)　at　different　hydrogen　surface　coverages　were　determined　by　ab　initio　thermodynamics.　The　phase　diagram　of　stable　H　coverage　on　each　plane　was　obtained　and　morphology　evolution　of　the　Co　nanocrystals　with　various　surface　hydrogen　coverages　was　predicted　by　the　Wulff　construction.　Addition　of　H-2　changes　the　facet　stability,　generating　diverse　morphological　Co　nuclei.　The　kinetic　role　of　H-2　in　adatom　Co　surface　diffusion　at　different　H　coverages　was　investigated　by　DFT.　The　results　suggest　that　surface　H　hinders　Co　surface　diffusions,　except　for　Co(100)　at　0.56　monolayer　coverage.　The　projected　density　of　states　gives　deeper　insight　into　the　electronic　structures　of　Co　adatoms　with　addition　of　the　surface　H　atoms,　which　affect　its　surface　diffusion　ability.　(C)　2019　Chinese　Society　of　Particuology　and　Institute　of　Process　Engineering,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.　All　rights　reserved.