ZnO films were deposited on glass and SiO2/Si substrate by RF magnetron sputtering technique using high purity ZnO target at various RF power. The structural properties of ZnO thin film deposited on glass and SiO2/Si substrate were studied. The structural properties of the films were carried out by the surface profiler and field emission scanning electron microscope (FESEM). It was found that the average grain size of ZnO increases with increasing RF power and ZnO deposited on SiO2/Si substrate with RF power between 50- 150 Watt gave the lower average grain size which is desired for the gas sensor applications.