In combination with scanning electron microscopy and x-ray pole-figure analysis high resolution electron microscope (HREM) observation of the diamond-silicon cross section in a 〈001〉 epitaxially oriented diamond film was carried out to investigate the atomic interfacial microstructure. The films were prepared by microwave plasma chemical vapor deposition using a bias-enhanced nucleation technique. Due to the multiple fit of diamond and silicon lattices, there is a periodic 3-to-2 registry of {111} atom planes of the epitaxial diamond-silicon interface. Planar defects on diamond {111} planes and interface misfit dislocations are shown for epitaxially oriented and for slightly misoriented diamond crystallites. A cubic silicon carbide ‘‘transition’’ is found to be unnecessary for the epitaxy. The misorientation of the grown crystallites is also studied and found to be probably due to interface imperfection.