The aim of this study is to obtain high-quality zinc oxide thin films by reactive radiofrequency (rf) magnetron sputtering. The thin films were prepared at constant total gas pressure, with different oxygen and argon contents. The ZnO samples were characterized by several methods. From XRD measurements it was confirmed that ZnO films are c-axis oriented, the line width and intensity are sensitive to O2/Ar gas pressure. All films exhibited excellent transmission (in excess of 70 %) in the visible range with a steep fall off in transmission at 425 nm. From the absorbance measurements the optical band-gap energy was extrapolated according the transmission spectrum. It shows that the optical band gap of the films increased from 3.233 eV to 3.288 eV with increase in the oxygen concentrations from 20 % to 70 %. Refractive indexes of the obtained thin films were carried out in this study.