Magnesium doped zinc oxide (MgxZni3O) thin films were synthesized on silicon substrate through sol-gel process. Mg0,15Z110,85 0 thin films were annealed at 500-800 C and ZnO, Mg0.1 ZI10,9 0, Mg0,05Z110,950 thin films were annealed at 600 C for 60 min, respectively. The results show that all the samples are of a hexagonal wurtzite structure of ZnO. The surface morphology is strongly dependent on mean grain size and surface fluctuation. Fourier transform infrared spectra reveal that the vibration peak at 420 cm-1 is of the intrinsic lattice absorption of ZnO. The peak at 1083 cm -1 belongs to Si O Si asymmetric stretching vibration. Photoluminescence spectra show that the ultraviolet emission (365-400 nm) and the broad visible emission (469-569 nm) are observed. In particular, Mg0.05Zno.950 thin film annealed at 600 C exhibits the highest photocatalytic activity, degrading MO by almost 85.8% after 180 min illumination. The photocatalytic activity of the thin film is a synergistic effect defined by grain size, roughness factor, oxygen defects and amorphous MgO. (C) 2014 Elsevier B.V. All rights reserved.