Cobalt　catalysts　have　great　potentials　towards　CO　hydrogenation.　Herein,　a　facial　H-2　pressure-controlled　strategy　was　developed　to　synthesize　two　typical　branched　cobalt　nanostructures:　nail-like　cobalt　and　urchin-like　cobalt.　The　H-2-controlled　over-growth　mechanisms　have　been　proposed,　and　supported　by　both　experiments　and　DFT　calculations.　The　over-growth　of　Co　nanocrystals　can　be　finely　controlled　by　the　selective　and　competitive　adsorption　between　H-2　and　OAm　molecules　on　cobalt　surface.　Both　nail-like　cobalt　and　urchin-like　cobalt　exhibit　superior　CO　hydrogenation　activity　to　ordinary　Co　nanoparticles,　due　to　their　HCP　phase,　unique　branch　structure　with　high　specific　area,　and　more　exposed　active　(1　0　-1　1)　crystal　facets.