Transmittance envelope of the thin film-substrate system and full spectra fitting method are two important methods to determine the optical constants of the optical thin films. Ion beam sputtering deposition technique was used to manufacture HfO2 single layer thin film onto fused silica substrate. The two methods were used to calculate optical constants of the HfO2 thin film in the extreme wavelength, and the Cauchy dispersion model was used to fit the optical constants in wavelength region from 300 nm to 1000 nm. Using the thin-film optical constants obtained above we calculated the spectral transmittance and judged the inversion accuracy of the two methods. The results show that the accuracy of the full spectra fitting method is higher than the transmittance spectra envelope. The similarities and differences between the two methods are also discussed in this paper.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.