The optical functions of AlN thin films deposited on the quartz substrates by the reactive radio frequency magnetron sputtering, such as refractive index, extinction coefficient, optical band gap, and film thickness were determined from the transmittance and reflectance spectra in the range of 190-820 nm. For these analyses, an inverse synthesis method was established after literature survey. The results were doublechecked with a modified envelope method, and compared with those of previous reports. Spectroscopic ellipsometry analyses were performed to confirm the accuracy of the methods. Refractive indices of AlN films in this study were in the range of 1.95-2.05 at 633 nm and 2.26-2.38 at 250 nm, depending on the preparation conditions. The extinction coefficients Mere small (<5x10(-4)) and nearly constant at low energy region (<2 eV), but exhibited various dispersion features at 2.2-3.5 eV, indicating different amount and kinds of defects of AIN films. The absorption coefficient at near-band-gap energy exhibited a strong thickness dependence, resulting in various optical band-gap energies ranging 5.34-5.71 eV.