Solar　light-driven　overall　water　splitting　for　hydrogen　production　is　an　ideal　solution　to　climate　warming　and　energy　shortage　issues.　Obtaining　a　highly　efficient　and　stable　photocatalyst　remains　a　major　challenge　at　present.　Herein,　NixP/gamma-Ga2O3　nanosheets,　which　were　synthesized　from　NiCl2,　NaH2PO2,　and　home-made　gamma-Ga2O3　nanosheets　by　the　photodeposition　method　under　254　UV　irradiation　for　30　min,　are　found　as　a　highly　active　and　durable　photocatalyst　for　pure　water　splitting　into　H2　and　O2　without　a　sacrificial　reagent.　The　H2　production　rate　is　as　high　as　5.5　mmol　center　dot　g-1　center　dot　h-1　under　125　W　high-pressure　mercury　lamp　irradiation,　which　is　3.4　and　2.5　times　higher　than　that　on　the　pristine　gamma-Ga2O3　nanosheets　and　Pt/gamma-Ga2O3　nanosheets,　respectively,　and　is　2.0　times　higher　than　that　on　the　0.5　wt　%　Ni2P/gamma-Ga2O3　reported　previously.　However,　the　O2　evolution　rate　is　much　less　than　the　H2　evolution　rate　in　the　initial　reaction　stage.　On　prolonging　the　irradiation　time,　H2　evolution　declines,　while　O2　evolution　increases　until　it　reaches　its　stoichiometric　value　corresponding　to　H2.　The　reason　for　the　photocatalytic　behavior　of　NixP/gamma-Ga2O3　is　studied　and　the　corresponding　mechanism　is　suggested.　The　absent　or　low　oxygen　evolution　in　the　initial　reaction　stage　is　because　the　dioxygen　generated　from　water　oxidation　by　the　photogenerated　holes　is　wholly　or　partially　captured　by　the　surface　oxygen　vacancies　to　form　the　surface　peroxide　bonds　(-O-O-).　Once　the　oxygen　vacancies　are　eliminated　by　the　photogenerated　O2,　the　overall　water　splitting　reaction　would　reach　the　steady　state.　Thereafter,　H2　production　decreases　from　5.5　to　2.0　mmol　center　dot　g-1　center　dot　h-1,　but　the　O2　evolution　gradually　approaches　the　corresponding　stoichiometric　value,　especially　for　the　photocatalyst　treated　with　H2O2　for　24　h.