The use of electron energy loss spectroscopy (EELS) in determining the electronic properties of Si and thickness dependent loss function of Si with 0.14 eV energy resolution were investigated. The dielectric response function was used to describe the interaction of a transmitted electron beam with the solid specimen. The specimen was oxidized in air for several days so that a thin amorphous SiO2 layer was formed covering both surfaces. It was observed that for fine structure investigations a high energy resolution of the electron source and the spectrometer were essential. The tails of the zero loss peak (ZLP) have also to be removed for extreme low loss spectroscopy.