Nanocrystalline ZnO thin films is a promising material for short-wave laser and luminescence etc,due to its wide band gap (3.37 eV) and high exciton binding energy(60 meV) at room temperature.The high-quality nanocrystalline ZnO thin films is prepared by using sol-gel,grow on quartz glass subtractes,Photoluminescence spectra and Absorption spectra of nanocrystalline ZnO thin films with excitation wavelength 330 nm are measured at room temperature. Two emission bands are observed,one being a strong and narrow ultraviolet emission band its central wavelength is 392 nm and the other being weak and wide green visible band its central wavelength is 519 nm. Crystal structure of samples are examined by X-ray diffraction (XRD) pattern,the mean grain size of nanocrystalline ZnO thin films are calculated by with Debye-Scherrer formula. The results indicate that nanocrystalline ZnO thin films has a hexagonal wurtzite structure and polycrystalline,and showed sharp diffraction peak for ZnO(002),which indicate that as-sputered film were highly c-axis oriented. The grain increases with annealing temperature,Absorption spectra of ZnO thin films indicated that the band gap of the thin films was 3.30 eV.Photoluminescence spectra showed that ZnO thin films emitted strong UV Photoluminescence. The intensities of PL peaks decrease with increasing annealing temperature. In particular the mechanism behind the visible luminescence has also been discussed in this paper. It should be said that the mechanism behind the visible luminescence is still a question to debate. In this paper the visible emission processes of nanocrystalline ZnO. The experiments prove that the luminescence emission peak located 519nm corresponds to the transition from the shallow level of oxygen vacancy to the shallow level of zinc vacancy or electron transition from deep level of oxygen vacancy to valence band.