Erbium oxide thin films were prepared at different Ar/O2 flow ratios by reactive radio frequency magnetron sputtering. The optical constant and band gap of the Er2O3 thin films were studied by using spectra ellipsometry and ultraviolet-visible spectra. The dielectric properties and current-voltage (I-V) measurements were carried out on various films. The results reveal that the refractive index, band gap and dielectric constants of the Er2O3 films increase with increasing Ar:O2 flow ratios, but the extinction coefficient does not change with that new ratio. The Er2O3 films deposited at flow ratios of Ar/O2 7∶1 present the best physical properties. The refractive index of the films is 1.81 in the wavelength range of 500～1000 nm. The band gap is 5.73 eV. The dielectric constants is 10.5.