Interface structures in monocrystalline beta‐silicon carbide thin films grown on (001) silicon substrates have been studied by high‐resolution electron microscopy of cross‐sectional specimens. Despite a large lattice mismatch, there is a periodic registry of {111} atom planes across the SiC‐Si interface. Planar defects on SiC {111} planes are grown‐in and arise primarily from lattice and thermal expansion mismatch. Thermal oxidation in wet atmospheres results in preferential attack of the SiC film at sites where planar defects intersect the film surface, whereas oxidation in dry atmospheres does not.