A curve fit method and fitting results of refractive indexes of Ta 2O 5 and SiO 2 thin films deposited by using ion beam sputtering in Fabry Perot multilayer systems used for optical telecommunication dielectric thin film filters (Mux/Demux) at wavelength 1550 nm are preseted. The sputtering time, deposition rate and optical thickness of each layers of the filters are given. At last, the measured results by means of curve fitting are discussed.