We　studied　the　structural　and　electronic　proper-ties　of　carbon　nanotubes　under　hydrostatic　pressures　based　on　molecular　dynamics　simulations　and　first　principles　band　structure　calculations.　It　is　found　that　carbon　nanotubes　experience　a　hard-to-soft　transition　as　external　pressure　increases.　The　bulk　modulus　of　soft　phase　is　two　orders　of　magnitude　smaller　than　that　of　hard　phase.　The　band　structure　calculations　show　that　band　gap　of　(10,　0)　nanotube　increases　with　the　increase　of　pressure　at　low　pressures.　Above　a　critical　pressure　(5.70GPa),　band　gap　of　(10,　0)　nanotube　drops　rapidly　and　becomes　zero　at　6.62GPa.　Moreover,　the　calculated　charge　density　shows　that　a　large　pressure　can　induce　an　sp(2)-to-Sp(3)　bonding　transition,　which　is　confirmed　by　recent　experiments　on　deformed　carbon　nanotubes.