The　adsorption　of　oxygen　on　the　GaN　surfaces　is　calculated　with　DFT　method　combining　a　cluster　model.　From　the　total　energy　minimization,　we　found　that　the　O-2　lying-down　orientation　are　favored　over　upright　orientation,　and　the　chemisorption　with　the　O-O　axis　parallel　along　the　bridge　site　on　the　GaN　(0001)　surface　is　the　most　stable.　The　electrons　transferred　from　the　substrate　to　the　O-2　molecule　occupy　the　O-2　antibonding　orbital,　thus　leading　to　the　bond　strength　weakened,　which　can　be　reflected　by　the　elongated　O-2　bond　length,　1.51　Angstrom,　compared　to　1.21　Angstrom　of　the　free　O-2.　The　energy　barrier　estimated　for　the　dissociation　of　O-2　at　the　bridge　site　on　the　GaN　(0001)　surface　is　0.7　eV.　The　dissociated　O　adatom　favors　the　three-fold　hollow　(fcc)　site　with　the　Ga-O　bond　length　2.14　Angstrom　on　the　(0001)　surface　and　1.90　Angstrom　on　the　(000　(1)　over　bar)　surface,　respectively,　similarly　with　the　bond　length　in　bulk　Ga2O3　ranging　from　1.8　to　2.1　Angstrom.　The　significantly　small　adsorption　height　(0.47　Angstrom)　of　the　atomic　O　on　the　(000　(1)　over　bar)　surface　make　us　believe　that　the　oxygen　will　be　more　easily　incorporated　on　this　surface,　which　means　that　the　oxidation　occurs　on　the　(000　(1)　over　bar)　surface　more　easily.　(C)　2004　Elsevier　B.V.　All　rights　reserved.