Effects of the O 2 /Ar flow ratio in the sputtering process on the crystallinity, surface roughness, carrier concentration, carrier mobility, and optical properties of Al-doped ZnO thin films deposited on sapphire (001) substrates by RF magnetron sputtering were investigated. XRD spectra showed a preferred orientation along the c-axis and a minimum FWHM of the (002) XRD intensity peak for the O 2 /Ar flow ratio of 0.5. The (101)peak also appeared and the degree of preferred orientation decreased as the O 2 /Ar flow ratio increased from 0.5 to 1.0. AFM analysis results showed that the surface roughness was lowest at the O 2 /Ar flow ratio of 0.5 and tended to increase owing to the increase of the grain size as the O 2 /Ar flow ratio increased further. According to the Hall measurement results the carrier concentration and carrier mobility of the fan decreased and thus the resistivity increased as the O 2 /Ar flow ratio increased. The transmittance of the ZnO:Al film deposited on the glass substrate was characteristic of a standing wave. The transmittance increased as the O 2 /Ar flow ratio in-RF magnetron sputtering increased up to 0.5. Considering the effects of the O 2 /Ar flow ratio on the surface roughness, electrical resistivity and transmittance properties of the ZnO:Al film the optimum O 2 /Ar flow ratio was 0.5 in the RF magnetron sputter deposition of the ZnO:Al film.