In this paper, high quality nanocrystalline ZnO film has been prepared by using thermal oxidation of ZnS thin film, which was grown on a Si substrate by the low pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The X-ray diffraction (XRD) patterns show that ZnO thin film has a hexagonal wurtzite structure with preferred (002) orientation. In photoluminescence (PL) measurements, a strong PL with a full width at half maximum (FWHM) of 10 nm (88 meV) around 380 nm at room temperature was observed from the samples annealed at 900 ℃. The PL intensity ratio of the UV emission to the deep-level emission is 28 at room temperature, providing evidence of the high quality of the nanocrystalline ZnO films.