To　investigate　the　effect　of　thickness　on　the　structural　and　optical　properties　of　niobium-doped　beta　gallium　oxide　(beta-Ga2O3:Nb)　thin　films,　a　series　of　beta-Ga2O3:Nb　thin　films　with　different　thicknesses　were　prepared　by　radio-frequency　magnetron　method.　The　crystalline　quality　is　highly　improved　when　the　film　thickness　exceeds　145　nm.　The　surface　exhibits　different　morphologies　under　different　film　thicknesses.　The　average　transmittance　of　all　the　films　are　over　80%　in　visible　range,　and　that　the　ultraviolet　absorption　edge　shifts　to　longer　wavelength　indicates　the　bandgap　shrinks　with　increasing　the　film　thickness.　Moreover,　the　photoluminescence　spectrum　measurements　indicate　that　fewer　defects　were　formed　when　the　film　thickness　is　around　145　nm.