Niobium-doped MgxZn1-xO (Nb-MZO) mixed oxide films with high transmittance were successfully deposited on sapphire substrates by a radio-frequency (RF) magnetron sputtering using a 4-in ZnO/MgO/NbOx (75/20/5 wt %) target. In this study, the films were analyzed through a Hall test, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmittance. The XRD results showed MgO2 (002) wurtzite peak as well as an MgxZn1-xO (111)-cubic peak. The absorption edges of these Nb-MZO films were located in the UV region, implying that the MgO content of the Nb-MZO layer increased the bandgaps. The XPS spectra of Nb-MZO films were also used to analyze the composition of the as-grown and annealed Nb-MZO films. These results indicate that the Nb-MZO films are ideal for use as transparent contact layers.