A new method for measuring the dielectric functions change with the thickness of nanometal thin films was proposed. To confirm the accuracy and reliability of the method, a nano-thin wedge-shaped gold (Au) film with continuously varied thicknesses was designed and prepared on K9 glass by direct-current-sputtering (DC-sputtering). The thicknesses and the dielectric functions in the wavelength range of 300-1100 nm of the nano-thin Au films were obtained by fitting the ellipsometric parameters with the Drude and critical points model. Results show that while the real part of the dielectric function (ϵ1) changes marginally with increasing film thickness, the imaginary part (ϵ2) decreases drastically with the film thickness, approaching a stable value when the film thickness increases up to about 42 nm. This method is particularly useful in the study of thickness-dependent optical properties of nano-thin film.