The　ammonia　adsorption　on　the　GaN　(0001)　surface　has　been　investigated　by　using　DFf/B3LYP　method　combined　with　the　cluster　model　approach.　The　dissociative　adsorption　of　NH2　and　His　found　thermodynamically　favored　relative　to　the　molecular　NH3　adsorption　by　decreasing　the　total　energy　of　0.95　eV.　The　adsorption　geometries　of　the　molecular　and　dissociative　NH3　are　given　in　detail,　among　which　the　molecular　NH3　bonds　to　the　surface　Ga　with　its　lone　electronic　pair,　and　the　N　atom　of　NH2　adspecies　forms　the　four-fold　coordinated　N　by　bridging　two　surface　Ga　atoms.