In this paper, we report the photoluminescence from high-quality nanocrystalline ZnO thin films. The high-quality nanocrystalline ZnO thin films are prepared by thermal oxidation of ZnS films at 800℃, which are deposited by low-pressure metal-organic chemical vapor deposition technique. X-ray diffraction indicated that the nanocrystalline ZnO thin films have a polycrystalline hexagonal wurtzite structure. A strong ultraviolet emission peak at 3.26 eV was observed and the deep-level emission band was barely observable at room temperature. The strength (ΓLO) of the exciton-longitudinal-optical (LO)-phonon coupling is deduced from the temperature dependence of the full width at half maximum of the fundamental excitonic peak. ΓLO is reduced greatly due to the quantum confinement effect.