Thin films of Ga-doped ZnO (GZO) were prepared on glass and Al2O3 (0001) substrates by using RF magnetron sputtering at a substrate temperature of 350 °C, RF power of 175 W, and working pressure of 6 mTorr. The effect of film thickness and substrate type on the structural and electrical properties of the thin films was investigated. X-ray diffraction study showed that GZO thin films on glass substrates were grown as a polycrystalline hexagonal wurtzite phase with a c-axis preferred, out-of-plane orientation and random in-plane orientation. However, GZO thin films on Al2O3 (0001) substrates were epitaxially grown with an orientation relationship of (00001)[112̄0]GZO (00001) [112̄0]Al2O3. The structural images from scanning electron microscopy and atomic force microscopy showed that the GZO thin films on glass substrates had a rougher surface morphology than those on Al2O3 (0001) substrates. The electrical resistivity of 1000 nm-thick GZO thin films grown on glass and Al2O3 (0001) substrates was 3.04 × 10-4 Ωcm and 1.50 × 10-4 Ω2cm, respectively. It was also found that the electrical resistivity difference between the films on the two substrates decreased from 9.48 × 10-4 Ωcm to 1.45 × 10-4 Ωcm with increasing the film thickness from 100 nm to 1000 nm.