Highly oriented polycrystalline ZnO films were deposited on Si substrate by rf reactive sputtering technique. X-ray diffraction (XRD), atomic force microscope (AFM) and the refractive index were employed to analyze the influence of the post-annealing treatment on the structural properties of ZnO thin films. It has been found that the grain size of ZnO thin films increases with increasing the annealing temperature, the shift of the diffraction peak position from its normal powder value was observed. AFM analysis shows that the surface roughness of ZnO films is very low at temperature between 250 and 600degreesC. The packing density investigation shows ZnO films can obtain high packing densities (above 0.973) in the annealing temperature rang from 450 to 600degreesC. (C) 2004 Elsevier B.V. All rights reserved.