High_quality ZnO films have been epitaxially grown on Si substrates by low_pressure metal organic chemical vapor deposition(LP_MOCVD) and two kinds of samples have been prepared:one is ZnO directly grown on Si while the other one grown with a SiC buffer layer.According to the results of Raman spectra,XRD,AFM,and photoluminescence,it is indicated that the tensile strain in the epitaxial ZnO films has a great influence on the crystalline quality of ZnO films,and the SiC buffer layer can effectively modulate the tensile strain,hence reduce the concentration of defects and improve the film quality.Then,from the defect formation mechanism,we suggest that for ZnO/Si,the large tensile strain results in high concentration of non_radiation recombination centers,which largely reduce the intensity of UV and green emissions; but for ZnO/SiC/Si,the small tensile strain results in oxygen antisite defects OZn in ZnO films,which enhance the green emis sion in photoluminescence.