The optical constants and thickness of Nb2O5 optical thin film deposited by a reactive magnetron sputtering technique have been retrieved by all measurement data of the normal incidence transmittance. A computer program written in FORTRAN for determining the refractive index, extinction coefficient and thickness of this film has been developed and tested on two examples. This method can also calculate the optical constants and thickness of the film with a thickness thinner than a quarter wavelength optical thickness. Moreover, the optical constants calculated by this method are more accurate than those calculated by the envelope method. We also suggest that this method should be able to apply to other optical films, such as TiO2, SiO2 and Ta2O5.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.