The　adsorption　of　atomic　hydrogen　on　(5,5)　armchair　carbon　nanotube　has　been　studied　by　using　density　functional　theory,　and　the　geometries　and　adsorption　energies　for　the　5%　and　10%　coverage　of　H　atoms　have　been　investigated.　The　calculated　results　indicate　that,　the　adsorption　of　H　at　the　exterior　is　energetically　favorable　than　that　at　the　interior,　and　the　second　H　atom　prefers　to　be　adsorbed　at　the　carbon　sites　near　the　first　one.　By　analyzing　the　band　structures,　it　is　known　that　after　the　adsorption　of　the　first　H　atom,　the　armchair　nanotube　will　become　semiconductor.　When　the　second　H　atom　is　adsorbed　at　the　even　site,　the　tube　keeps　its　conductive　characteristic,　while　the　transfer　ability　of　this　tube　is　weaker　for　the　case　at　odd　site.　The　change　of　the　transport　property　can　be　explained　by　examining　the　arrangement　of　the　pi　conjugation　of　the　carbon　nanotube.