Monoclinic　β-Ga2O3　nanosheets　hold　great　potential　applications　in　electronic,　optical,　and　photocatalytic　fields.　In　this　study,　two-dimensional　β-Ga2O3　nanosheets　were　successfully　fabricated　through　a　simple　crystalline　phase　transition　from　the　as-prepared　ultrathin　γ-Ga2O3　nanosheets.　The　photocatalytic　hydrogen　evolution　reaction　under　UV　light　irradiation　was　achieved　on　the　two　kinds　of　photocatalysts.　However,　β-Ga2O3　with　a　higher　crystallinity　shows　a　lower　photocatalytic　activity　in　comparison　with　γ-Ga2O3.　The　average　apparent　quantum　yield　is　calculated　to　be　0.29%　for　β-Ga2O3　nanosheets　and　1.82%　for　γ-Ga2O3.　More　efficient　separation　and　transfer　rates　of　photogenerated　carriers　and　larger　specific　areas　were　found　in　γ-Ga2O3.　On　the　basis　of　the　analysis　of　the　structures　of　γ-Ga2O3　and　β-Ga2O3,　it　is　proposed　that　the　disordered　or　defective　structure　contributes　to　the　improvement　of　photocatalytic　activity　to　some　extent.　Therefore,　it　is　significant　to　develop　the　photocatalyst　with　a　stable　structure　and　a　certain　number　of　defects　at　the　same　time.　©　2018　American　Chemical　Society.