ZnO:Al thin films were deposited on sapphire(001) substrates by RF magnetron sputtering. Effects of the O2/Ar flow ratio in the sputtering process on the crystallinity, surface roughness, carrier concentration, carrier mobility, and optical properties of the films were investigated. AFM analysis results show that the surface roughness is lowest at the O2/Ar flow ratio of 0.5 and tends to increase owing to the increase of the grain size as the O2/Ar flow ratio increases further than 0.5. According to the Hall measurement results the resistivity increases as the O2/Ar flow ratio increases. The transmittance of the film tends to increase as the O2/Ar gas flow ratio increases up to 0.5 but it nearly does not change with continued increases in the O2/Ar flow ratio. Considering the effects of the the O2/Ar flow ratio on the surface roughness, electrical resistivity and transmittance properties of the ZnO:Al film the optimum O2/Ar flow ratio is 0.5 in the RF magnetron sputter deposition of the ZnO:Al film.