The　geometries　and　electronic　structures　of　three　kinds　of　(5,5)　armchair　carbon　nanotubes　with　open　or　capped　ends　were　studied　by　PM3　method.　The　results　indicate,　as　the　length　of　the　tube　increases,　in　addition　to　the　periodical　variation　of　the　energy　gap　between　the　HOMO　and　LUMO,　the　geometries　of　tube　also　exhibit　oscillation　and　can　be　divided　into　three　groups.　For　die　nanotubes　belonged　to　the　same　group,　their　geometries　and　the　electronic　structures　are　very　similar.　For　the　cap　of　nanotube,　it　can　be　seen　as　the　prolongation　of　the　tube　wall,　therefore,　the　electronic　structures　of　tube　with　capped　ends　are　close　to　the　tube,with　open　ends.　Furthermore,　the　surface　states　located　at　the　ends　of　nanotube　were　also　discussed　in　this　paper.