We used cross-sectional high-resolution transmission electron microscopy to directly observe the atomic structures at SiO2/Si and Si3N4/Si interfaces. These observations provided the first direct evidence that cristobalite-like crystalline SiO2 exists at the interface. Our observations also show that in one of the observed N-induced interfacial geometries, a N atom replaces a Si atom at the second interfacial layer, and a dangling bond was produced on a Si atom adjacent to the N atom. We thus argue that a large elastic strain at a crystalline SiO2/Si interface probably plays an important role in preserving the initial atomic-scale flatness of the interface; that is, the strain results in layer-by-layer oxidation. Furthermore, we infer that the difference in interfacial flatness between Si3N4/Si and SiO2/Si can be explained in terms of the difference in their interfacial strain. © 2003 American Vacuum Society.