Plasma assisted molecular beam epitaxy was employed to prepare ZnO thin films on Si(111) substrate. In order to look for the optimal substrate temperature for growth of ZnO thin films, the dependence of the quality of ZnO thin films on substrate temperature ranging from 350℃ to 750℃ was studied by X ray diffraction(XRD) and photoluminescence(PL) spectra analysis. The full width at half maximum (FWHM) of XRD and PL spectra become narrower and then wider, a preferred oriented ZnO thin film on Si substrate was obtained at 550℃. The PL spectra show a strong ultraviolet(UV) band emission with a weak deep level emission. The origin of the UV band is from free excition recombination, testified by the temperature dependent PL spectra in the rang of 81K～300K.