Mg-doped ZnO films were deposited using radio frequency reactive magnetron sputtering at different ratios of oxygen to argon gas flow. The crystal structures, surface morphology and optical properties of Mg-doped ZnO thin films were analyzed. The results indicated that three main emission peaks located at 400, 440 and 483 nm were observed in Mg-doped ZnO films. Violet peak at 400 nm and blue peak centered at 440 nm shifted to 392 nm and 422 nm, respectively, as the ratio of oxygen to argon gas flow is increased. The spectra shift mechanism was discussed, which would be caused by small amount of Zn nanoparticles in Mg-doped ZnO films. The electrons on the Zn conduction band will go across the interface for energy equilibration, and then transition to Zn vacancies and the top of the valence band, which may cause spectra shift. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.