Mg-doped zinc oxide (ZnO:Mg) thin films have been deposited by radio frequency(RF)magnetron sputtering,using a metallic Zn target and MgO ceramics target in O_2and Ar mix ture atmosphere.Effects of O₂/Ar on crystalline structure,surface morphology a nd optical properties of ZnO:Mg thin films are investigated. The results show that the thin films deposited at room temperature (RT) are s ignal phase ZnO (002) in the O₂/Ar range of 1∶1－3∶1,and thin films show ch aracteristics of 3D nucleation and growth.Post deposition annealing of thin film was carried out at 200°C in N_2atmosphere.The transmi ssion spectrum of annealed thin film deposited at the O₂/Ar of 3∶1exhibits high transmittance in the wavelen gth region of 380－1200nm.The average transmittance is ～85%,and the maximum transmittance is 90% in the visible regi on.The optical band gap (Eg) of thin film is 3.51eV,and the E_g value can be modula ted by Mg-doping in ZnO:Mg thin film.Moreover,the refractive index and thickness of thin film are calculated based on the envelop e curves of the optical transmission spectrum.The thin film exhibits the refractive index of 1.963(at 589.3nm) and thickness of 285nm.