Using the reactive radio-frequent magnetron sputtering method, ZnO thin films were deposited on pretreated (100) sapphire substrates. The effect of substrate pretreatments on the growth of ZnO films was studied with the structural and morphological characterization using atomic force microscopy, reflection high-energy electron diffraction and X-ray diffraction. It was found that both the original substrate and the substrates annealed in vacuum (8×10-4 Pa), nitrogen atmosphere (40 Pa), and oxygen atmosphere (40 Pa) at 750℃ for 4h have the same surface structure that can be indexed to be α-Al2O3 (001). The surface morphologies of the substrates are different from each other. All the ZnO films grown on the substrates are highly c-axis textured. Their morphologies, however, are related to the pretreatment. For the substrate annealed in vacuum, the film has the morphology with -c and +c epitaxial islands, similar to ZnO grown on the untreated substrate. For the substrate annealed in nitrogen, the film has the morphology with only -c epitaxial islands and has quite large grains and surface roughness. For the substrate annealed in oxygen, the films also has the morphology with -c epitaxial islands, but the surface is much smoother. The surface roughness is as small as 15nm.