Monocrystalline ZnO films with mosaic structure are successfully grown on Si(111) substrate by atmospheric-pressure metal-organic chemical vapor deposition (MOCVD). Low-temperature AlN buffer layer is introduced between the films and substrate to protect the substrate from being oxidized and eliminate the thermal and crystals lattice mismatch. Only appear diffraction peaks of Si(111) plane, ZnO(000l) index planes and AlN (000l) index planes [including the (0002) and (0004) planes] in double-crystal X-ray diffraction 2θ/ω scanning curve. The lattice constant of C direction is 0.5159 μm, which indicates ZnO film is suffering from the tensile stress. The full wide of half maximum (FWHM) of ZnO (0002) and ZnO (1012) double-crystal X-ray diffraction ω-scan peaks are 460″ and 1105″ respectively. The crack density of ZnO surface is only 20 cm-1 by optical microscope graph determination. Mean square root roughness is 1.5 nm from the 3 μm×3 μm atomic force microscope scanning graph. The in-situ laser reflectance trace shows that a quasi-two-dimensional growth mode is obtained and the growth rate is 4.3 μm/h. Free exciton emission and binding exciton emission accompanied by their longitudinal optical phonon replicas are observed from the photoluminescence (PL) spectrum at 10 K. All the results show that AlN buffer layer is an effective route to obtain high quality ZnO film on Si (111) substrate by MOCVD.