In-plane lattice spacing oscillation was observed as functions of surface coverage and growing film thickness by reflection high-energy electron diffraction during the growth of strained BaTiO3 and unstrained SrTiO3 epitaxial layers on SrTiO3(100). For BaTiO3/SrTiO3, the oscillation continued until the critical thickness is reached, whereas for SrTiO3/SrTiO3, oscillation was observed only at the initial few monolayer growth. The origin of oscillations is discussed in relation to the non-tetragonal elastic distortion occurring at the free edges of 2D single monolayer islands. The amplitude of the oscillation strongly depended on the island size and density. (C) 2002 Elsevier Science B.V. All rights reserved.