Here,　niobium-doped　monoclinic　gallium　oxide　thin　films　of　different　thicknesses　were　deposited　on　p-Si　(100)　and　quartz　substrates　by　radio-frequency　magnetron　sputtering.　All　films　were　annealed　in　argon　ambient.　The　crystal　structure　and　surface　morphology　of　the　films　were　researched　using　x-ray　diffraction　and　scanning　electron　microscopy.　Then,　their　crystallite　size　was　evaluated　via　the　Debye-Scherrer　formula.　The　results　demonstrated　that　the　films　had　a　good　crystal　structure　and　a　flat　surface　when　the　thickness　was　around　300　nm.　The　films＇　optical　properties　were　also　investigated,　and　the　results　showed　that　all　of　the　films＇　transmittance　is　above　80%　to　ultraviolet-visible　light　whose　wavelength　is　above　350　nm.　Meanwhile,　the　films＇　optical　band　gap　decreased　as　their　thickness　increased.　The　Urbach　energy　of　all　films　was　calculated　by　the　Urbach　rule,　and　the　results　indicated　that　the　best　crystal　quality　occurred　when　the　thickness　was　around　300　nm.　The　films＇　electrical　characteristics　showed　that　the　current　was　larger　when　the　thickness　was　around　300　nm　and　that　the　contact　between　the　Au　electrode　and　films　was　Ohmic　contact,　independent　of　the　film　thickness　and　test　conditions.　These　findings　will　provide　useful　information　for　the　practical　application　of　Nb-doped　beta-Ga2O3　thin　films.