ZnO thin films were deposited by reactive cathodic radio-frequency (RF) sputtering from a pure Zn target in a gas mixture of 30% O2 and 70% Ar and at different RF powers. The structural properties of the as-deposited thin films were studied by X-ray diffraction (XRD). The optical properties (especially the refractive index, absorption coefficient and optical band gap) were investigated by optical transmission measurements in the ultraviolet-visible-near Infrared wavelength range. The XRD patterns showed that the as-deposited ZnO thin films are polycrystalline. The crystallite size varied with RF power reaching a maximum at 200 W. These results were correlated with X-ray refectometry measurements which revealed a minimum in the film density at 200 W. The deposition rate of these films varied from 2.53 to 5.27 nm/min depending on the RF-power, with a maximum at 200 W. On the other hand, the optical band gap Eg was quasi-constant (about 3.28 eV) when the RF power was increased from 100 to 300 W.