ZnO thin films are prepared by sol-gel method on Si substrates. The structural and optical properties of the films annealed at different temperatures are analyzed by X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence. The results of XRD show that each of all the ZnO thin films has a wurtzite phase and is preferentially oriented along the c-axis direction. The sample annealed at 900℃ exhibits a better crystalline quality. Bright and stable structured green luminescence is achieved from the Cu-doped ZnO thin film. The intensity of the green emission increases significantly after annealing at 800℃, while starts to decrease with further increasing temperature. Green luminescence is correlated with the creation of Zn vacancies. Green emission peaks are found to be dependent on the relative concentration of defect centers. The substitution of Cu2+ by Cu+ will increase concentration of defects in the Cu:ZnO thin film and result in very strong green emission.