At room temperature, ZnO thin films are deposited on quartz substrate by RF magnetron sputtering with 99.99% purity ZnO ceramic target as the sputtering source. An investigation has been made on the effects of sputtering power (120~180 W, power step size 20 W) on the microstructure and optical properties of ZnO thin films by X-ray diffractometer (XRD), thin-film thickness gauge, UV-visible spectrophotometer, and X-ray fluorescence spectrometer. The results show that the (002) plane diffraction peak appears near 34.3°, showing the c-axis preferred growth and wurtzite structure. With the increase of power, the refractive index of the films change in varying degrees. The average transmittance in the ultraviolet region of the wavelength range of 190~368 nm is less than 10%, while the average transmittance in the visible region is higher than 90%, with a steep absorption peak near 368 nm. The results show that the ZnO thin films are characterized with the largest grain size, good grain orientation, compact structure and the best crystal quality and optical properties with a the sputtering power of 160 W, an argon pressure of 0.5 Pa, the flow rate of argon being 8.7 mL/min，and deposition time being 60 min.