Defect structures of stoichiometric and nonstoichiometric perovskite thin films epitaxially grown on (100)SeTiO3 Substrates by pulsed laser deposition were studied using transmission electron microscopy and X-ray diffraction. The major defects in the partially relaxed stoichiometric BaTiO3 thin films are misfit dislocations with Burgers vectors of the type a <100>, threading dislocations connected to that dislocation, and inclined threading dislocations with Burgers vectors of the type a <110>. On the other hand, nonstoichiometric perovskite films show quite unique defect structures. A-site-excess BaTiO3 thin filius were epitaxially grown with a Ruddlesden-Popper fault on the (100) plane and a multiple nano-twin lamella. (C) 2002 Elsevier Science B.V. All rights reserved.