MgZnO thin films with a preferred orientation with the c axis perpendicular to the substrates was prepared on quartz substrates by radio frequency magnetron sputtering. The analysis of X-ray diffraction (XRD) and transmission electron microscope (TEM) indicated that the MgZnO polycrystalline films had hexagonal wurtzite single-phase structure. The TEM image of MgZnO polycrystalline film showed the tendency of layer growth. Sharp absorption edge appeared in the transmittance spectrum of the MgZnO film, the fundamental band gap of the MgZnO film was estimated to be 3.66 eV. The first-order Sellmeier dispersion relationship of MgZnO thin film was achieved by calculating of transmittance spectrum. With the increasing of wavelength, the refractive index decreased. The difference was only 1.6% between the values obtained by calculating and testing.