The fractional-dimensional space approach and analysis of spectroscopic ellipsometry are used to study an oxide overlayer effect on the measurement of the parameters of interband critical point (CP) of Si by a Si-SiO＿2 model. There sults of the calculation show that the oxide overlayer effect can influence the calculated CP parameters derived from an optical spectrum by decreasing t he ampl itude and dimensionality, and increasing the lifetime broadening. Moreover, the effect is enhanced with increasing thickness of the overlayer. However, this eff ect on threshold energy is shown to be very small and can be ignored.