Microstructural properties are found to be variant in the BaTiO3 (- x) films grown on SrTiO3(001) substrate under various oxygen pressures from 2 x 10(-2) Pa to 2 x 10(-5) Pa by laser molecular-beam epitaxy. Transmission electron microscopic studies reveal that the predominant defects in the films change from threading dislocations into (111) planar defects (i.e. stacking faults and nanotwins) by lowering the oxygen pressure. High density of these defects was observed in the BaTiO3 (-) (x) film prepared at the oxygen pressure of 2 x 10(-5) Pa, which shows metallic behavior. The relationships between oxygen pressure, microstructure, and electrical properties are established on the basis of oxygen deficiency. The formation of nanotwins in highly oxygen-deficient BaTiO3 - x epitaxial thin films results from accommodating excess oxygen vacancies induced by lowering oxygen pressure. (C) 2009 Elsevier B.V. All rights reserved.