An X-ray diffraction (XRD) simulation software based on the supercell model has been developed, which can calculate X-ray rocking curves of layered film/substrate systems by considering the attenuation of the amplitude of an X-ray beam due to an absorption effect. Considering the changes in the lattice constants along the thickness direction of LiNbO3/LiNbO3 (LN/LN) epitaxial films grown by the liquid-phase epitaxial (LPE) method, the X-ray rocking curves are simulated, and then the concentration profiles of Li are estimated. The Li content increases gradually from the interface towards the film surface, and then is saturated at a constant value. The saturation value is independent of the film thickness, whereas the thickness of the compositional fluctuation layer increases with increasing film thickness. It is concluded that the LN layers with nearly stoichiometric composition grow at the solid/liquid interface throughout the epitaxial growth, while the thickness of the fluctuation layer increases. The strained structures of the LN/LN films are attributable to the increase in the thickness of the fluctuation layer. (C) 2002 Elsevier Science B.V. All rights reserved.