Controlling　the　creation　of　oxygen　vacancies　can　effectively　regulate　the　optical　and　electronic　properties　of　metal　oxide　nanomaterials.　Over　the　past　several　decades,　numerous　metal　oxides　with　oxygen　vacancies　have　been　developed.　However,　an　investigation　about　oxygen　vacancies　leading　to　the　formation　of　nanosheets　with　different　thicknesses　has　not　been　available　up　to　now.　Here,　we　report　the　oxygen　vacancy　modulated　formation　of　γ-Ga2O3　nanosheets　and　demonstrate　that　the　thickness　of　the　nanosheets　is　not　the　decisive　factor　in　the　photocatalytic　hydrogen　evolution　reaction　of　ultrathin　2D　nanosheets.　Detailed　structural　characterization　indicated　that　γ-Ga2O3　prepared　at　160　°C　(γ-160)　with　a　morphology　of　ultrathin　nanosheets　possesses　the　highest　oxygen　vacancy　concentration　and　an　optimal　thickness　of　the　nanosheets.　The　enhanced　photocatalytic　performance　could　be　determined　from　the　synergistic　effects　between　the　ultrathin　2D　structure　and　the　O-vacancies　confined　in　the　ultrathin　nanosheets.　This　work　provides　an　efficient　strategy　to　regulate　the　formation　of　nanosheets　at　the　atomic　scale　and　enrich　the　study　on　the　effect　of　oxygen　vacancies　in　the　photocatalytic　water　splitting　reaction.　©　2018　The　Royal　Society　of　Chemistry.