The roughness at the Si-SiO2 interface has been determined by the ellipsometric spectroscopy. Several geometric forms have been used to simulate the surface irregularities. The dielectric properties of the rough layer are modelled in the effective-medium approximation by dividing the rough layer into a number of sub-layers with equal thickness. The asperity height H (normal to surface) and the correlation length L (along the surface) can be defined by best fit of the theoretically calculated multiphase reflective ellipsometric parameters ψcal, △cal to the measured ellipsometric parameters ψexp, △exp. It is found that the trend of the electric properties of the sample is consistent with the variance of its roughness. It is also found that the dielectric function of Si has some anisotropic properties at the interface.