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　O2　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　O2　molecule　occupy　the　O2　antibonding　orbital,　thus　leading　to　the　bond　strength　weakened,　which　can　be　reflected　by　the　elongated　O2　bond　length,　1.51Å,　compared　to　1.21Å　of　the　free　O2.　The　energy　barrier　estimated　for　the　dissociation　of　O2　at　the　bridge　site　on　the　GaN　(0001)　surface　is　0.7eV.　The　dissociated　O　adatom　favors　the　three-fold　hollow　(fcc)　site　with　the　Ga-O　bond　length　2.14Å　on　the　(0001)　surface　and　1.90Å　on　the　(0001̄)　surface,　respectively,　similarly　with　the　bond　length　in　bulk　Ga　2O3　ranging　from　1.8　to　2.1Å.　The　significantly　small　adsorption　height　(0.47Å)　of　the　atomic　O　on　the　(0001̄)　surface　make　us　believe　that　the　oxygen　will　be　more　easily　incorporated　on　this　surface,　which　means　that　the　oxidation　occurs　on　the　(0001̄)　surface　more　easily.　©　2004　Elsevier　B.V.　All　rights　reserved.