The growths of BaTiO3 films on interfacial SiO2 were carried out with molecular beam epitaxy (MBE) in various oxygen partial pressures. In the BaTiO3 growth on 2 nm-thick SiO2 film at the oxygen partial pressure of less than 4.8 x 10(-5) Pa, concavities at the BaTiO3 film surface and the Si/SiO2 interface appeared and the Si atoms diffused from the Si substrate to the top BaTiO3 layer. These were observed only after the film growth in the low oxygen pressure and only on ultrathin interfacial SiO2. And the silicide material was formed within the concavity. These results indicate that 2 nm-thick SiO2 film is not enough to the suppression of the reaction of the metal atoms with Si substrate. (C) 2003 Elsevier Science B.V. All rights reserved.