Abstract: Mg_xZn_1-xO alloy thin films were fabricated on c-plane sapphire （Al₂O₃） substrates by plasma assisted molecular beam epitaxy （P-MBE）. The films with different composition x, which is changed from 0 to 0.20, keep wurtzite crystal structure measured by X-ray diffraction （XRD）. Reflection high-energy electron diffraction （RHEED） and X-ray double crystal diffraction spectra show that the samples are single-crystal films. As x value is increased from 0 to 0.20, the full width at half maximum （FWHM） of （002）-oriented ZnO X-ray rocking curve is broadened from 0.249° to 0.708° and the lattice constant of c-axis is decreases from 5.205Å to 5. 185Å. The absorption edges of alloy thin films show the blue shifts with increasing x value in the transmission spectra at room temperature （RT）. Photoluminescence （PL） spectra at RT exhibit an intense ultraviolet emission, which shifts to high-energy side with increasing x values, and the deep level emission are not observed. The above results indicate that high quality single-crystal MgZnO thin films are fabricated by P-MBE.