The structure of interface of thermal silicon oxide on p- and n-silicon with different content of activators (boron and phosphorus) was studied by using the method of the local cathodoluminescence (CL). The results of the CL study of the thick silicon oxide layers on silicon show that the content of the defects related with oxygen deficit decreases near the interface. In the same time, new bands in green and red range appear in CL spectra. The CL spectra of the layer with thickness 5-15 nm near interface are analogous to CL spectra of a composite of silicon nanoclusters and silicon oxide. The comparison of CL spectra of silicon oxide grown on p- and n-silicon shows that the film on p-silicon is characterized by higher concentration of silicon-deficit defects and silicon "islands" near the surface. It may be the cause why the electrical hardness of silicon oxide on p-silicon is lower than the one on n-silicon. The integral electro-physical characteristics of silicon oxide also were measured. The bulk charge and the density of interface states of silicon oxide on p-silicon are higher than for oxide on n-silicon. The oxidization of n-silicon with nanostructure surface leads to the appearance of the CL bands related with oxygen deficit and silicon "islands" in silicon oxide. (C) 2004 Published,by Elsevier B.V.