Tin oxide thin films were deposited by rf reactive sputtering and annealed at $400^{circ}C$ for 1 h in vacuum. To minimize the influence such as reduction, oxidation, and doping on tin oxide thin films during annealing, a vacuum ambient annealing was adopted. The structural, optical, and electrical properties of tin oxide thin films were characterized by X-ray diffraction, atomic force microscope, UV-Vis spectrometer, and Hall effect measurements. After vacuum annealing, the grain size of all thin films was slightly increased and the roughness ($R_a$) was improved, however irregular and coalesced shapes were observed from the most of the films. These irregular and coalesced crystal shapes and the possible elimination of intrinsic defects might have caused a decrease in both carrier concentration and mobility, which degrades electrical conductivity.