We investigated the preparation of Ga2O3 thin films using a wet-process and the Sn-doping conditions to improve resistivity. Undoped and Sn-doped single layer Ga2O3 thin films were prepared by changing the solution concentration, the annealing time, the temperature rise rate, and the rotational speed during deposition. As a result, they were deposited uniformly with good crystallinity and transmittance and relatively small surface roughness. To improve the resistivity by another Sn-doping method, we prepared bilayer samples consisting of undoped Ga2O3 and SnO 2 layers in which Sn atoms can diffuse into the Ga2O3 layer by annealing: the drive-in diffusion method. Although the resistivity was changed, XPS and TEM-EDS measurements revealed that Al diffused from the sapphire substrate, indicating a mixed β-(Ga,Al)2O3 was prepared. By varying the annealing temperature to 800°C, Al diffusion was suppressed and pure Ga2O3 thin films were prepared. After investigating various atmosphere sintering conditions, we found that the samples sintered in a nitrogen atmosphere for both Ga2O3 and SnO 2 showed the lowest resistivity.