A single crystalline anatase TiO2 film was grown on a LaAlO3 substrate. Photocatalytic reduction of Ag ions resulted in Ag precipitation on the surface of the film. The density and the morphology of the Ag precipitation varied at the twin boundaries of the substrate. The anatase film should suffer from the strain caused by this twinning and the strain distribution should be different in each twin domains. Hence, this twin-domain dependent Ag precipitation is a possible clue for the electron-hole charge separation centers induced by the stress concentration in the anatase film which act as active sites for photocatalytic reaction. © 2005 Elsevier B.V. All rights reserved.