The structure and composition of the native Si oxide were studied using high depth resolution medium energy ion scattering (MEIS) spectrometry. The analysis revealed that the oxide is an amorphous material of thickness 20 Å. The results showed qualitatively that the interface between the oxide and the underlying structure consists of layers of Si atoms displaced from their normal lattice sites. The composition of the native Si oxide varies with depth. The region near the surface is more highly oxidized than that near the interface. However, based on the assumption that there are two nonregistered Si layers in the interface, the results showed that the oxide consists of a layer of stoichiometric SiO2 of thickness ∼ 7 Å and a layer of suboxide (SiOx, x < 2) of thic ∼ 6 Å.