Spectroscopic ellipsometric data, of which incident-light wavelength ranges from 3500 A to 6500 A, has been analyzed to determine the optical response of the interface between Pt and n-Si(111). The result shows an interface layer of thickness d2=30± 1 A, whose optical and dielectric properties are very different from the ones of the Pt film or the Si substrate. At the same time, the apparent optical constant spectra and dielectric function spectra of the interface layer were obtained in this work. A calculation based oil the effective medium theory (EMT) was done to fit the dielectric function spectra of the interfaoe layer. The resnlt indiDates the existence of Si-rich oxide in the interface layer, which can be ascribed to tlie reduction of the natiye Si oxide oil the suxfaoe of Si substrate caused by Pt. The restil-fc also suggests ihat the interface layer is a physical and cLemioal mixing region of Pt, Si and Si-rioh oxide.