Ga doped ZnO (GZO) films were prepared by radio frequency (rf) magnetron sputtering on glass or silicon substrates. Electrical, optical, and structural properties of these films were analyzed in order to investigate their dependence on thermal annealing temperature. GZO films with a minimum resistivity of 5.2×10-3 Ω-cm annealed at 400°C and a transparency above 80% in visible region were observed. The temperature-dependent conductivity affected the carrier transport and was related to the localization of carriers. The results of transmission spectra were consistent with the results of atomic force microscopy (AFM) scan. X-ray diffraction analysis and electron spectroscopy for chemical analysis were also used to investigate the properties of GZO films.