The　adsorption　and　decomposition　of　H2O　on　GaN(0001)　surface　have　been　explored　employing　density　functional　theory　(I)FT).　Two　distinct　adsorption　features　of　H20　on　GaN(0001)　corresponding　to　molecular　adsorption　and　H-OH　dissociative　adsorption　are　revealed　by　our　calculations.　The　activities　of　the　surface　reactions　of　H20　on　GaN(0001)　surface　are　investigated.　For　the　stepwise　processes　of　H2O　decomposition　into　H-2　in　gas　phase　and　adsorbed　O　atom　(H2O(g)　-＞　H2O(chem)-＞　OH(chem)　+　H(chem)-＞　2H(chem)　+　O(chem)-＞　H-2(g)　+　O(chem)),　the　first　and　second　steps　are　facile　and　can　even　occur　at　room　temperature;　while　the　last　two　have　high　barriers　and　thus　are　difficult　to　proceed,　especially　the　fourth　step　is　endothermic.　In　short,　H2O　adsorption　and　decomposition　into　H-2　in　gas　phase　and　adsorbed　O　atom　on　GaN(0001)　surface　are　exothermic　by　-43.98　kcal/mol.