Abstract: The ultra-thin silver films have been widely used in filters, high reflectivity mirror, and etc. The optical constants of ultra-thin silver films have severe impact on the characteristics of films. In order to investigate the optical constants of ultra-thin silver films, a series of different thickness silver films between 5.3 nm~26 nm were deposited on Si and glass substrates by thermal evaporation at room temperature, the thicknesses were measured by TalySurfCCI non-contact surface profiler, the optical constants n and k were measured by ellipsometer. Respectively, the thicknesses of ultra-thin silver films were 5.3 nm, 7.9 nm, 14.1 nm, 26.0 nm. The results show that the optical constants of ultra-thin silver are different from bulk silver. For the thickness of less than 14.1 nm, the refractive index n increases with the wavelength increasing from 380 nm to 600 nm, and decreases slowly then tends to a stable value with the wavelength increasing from 600 nm to 1 600 nm, while the extinction coefficient k increases with the wavelength increasing from 380 nm to 500 nm, then, slows down, tends to 0 and unchanged. When the film thickness is greater than 14.1 nm, the refractive index increases with increasing wavelength, and the extinction coefficient increases linearly with the wavelength.In generally, the refractive index decreases when the film thickness increases and tends to be that of bulk material, while the extinction coefficient k increases following the increase of thickness and tends to the bulk silver in the end. The optical constants fitted by this method were substituted into the TFc film system design software, and the transmittance was calculated. It is found to be in good agreement with the transmittance measured by the spectrophotometer.