Epitaxial LiNbO3 films have been deposited on (0001) Al2O3 substrates by pulsed laser deposition. The microstructure of the samples has been investigated by X-ray diffraction reciprocal space mapping. Data acquisition is performed on a high-resolution Set-up suited to the study of oxide materials. The (0001) planes of the layer are found parallel to the (0001) planes of the substrate. The large in-plane misfit gives rise to two in-plane orientations. In the direction perpendicular to the surface the crystallite size (163 nm) is much smaller than the film thickness (510 nm) suggesting the presence of planar defects. Moreover, the values of the mosaicity (1.2degrees) and root-mean-squared strain (0.17%) cannot be solely explained by misfit dislocations. Conversely the high-thermal expansion coefficient mismatch is at the origin of a high compressive strain level (-0.18%) and may also be responsible for the large column length distribution, root-mean- squared strain and mosaicity. (C) 2003 Elsevier Science B.V. All rights reserved.