The near-interface region of an epitaxial Ba 0.3 Sr 0.7 TiO 3 thin film grown on LaAlO 3 was found to consist of a high density of stacking faults bounded by partial dislocations. The stacking faults can extend over large distances . Various possible atomic configurations of the faults were considered. The atomic structures of the faults were identified using high-resolution electron microscopy and simulation as well as energy-filtered imaging. The and faults were found to lie predominately on the (...) {100} and {110} planes. The faults on , or have never been observed before in perovskites. The stacking faults on {100} have a structure consisting of a double layer of edge-sharing TiO 6 octahedra. The excess of Ti was detected by energy-filtered imaging. The formation of the extended stacking faults is probably related to a small amount of excess Ti during the film deposition, which may originate from the non-stoichiometry of the ceramic targets BaTiO 3 and SrTiO 3 . It is also enhanced by the misfit-induced compressive strain in the early stages of the film growth. (shrink)