GaN layers grown by metalorganic vapor phase epitaxy on sapphire substrates in epitaxial lateral overgrowth (ELOG) mode were studied using the high-resolution X-ray diffraction technique in double and triple axis modes (including reciprocal lattice mapping), both in reflection and transmission geometries. GaN layer grown on unmasked sapphire was examined in the same way for a comparison. We have found that in the fully overgrown ELOG structures: (i) hexagonal unit cells are deformed, so the lattice parameters a(perpendicular to) (perpendicular to the stripes) and a(30) (inclined by 30degrees to the stripes) are smaller by up to 0.1% and 0.02%, respectively, as compared to those in the GaN buffer, (ii) curvature radii of the samples are of 1.2-1.4m in the direction parallel to the stripes and of I m in the direction perpendicular to the stripes, which is consistent with the strain observed, and (iii) sapphire miscut along the [11.0] direction (i.e. perpendicular to the stripes) causes an additional inclination of ELOG lattice planes with respect to those in the buffer. A qualitative explanation of these findings is presented taking into account increase of GaN grain size in the ELOG layers. (C) 2002 Elsevier Science B.V. All rights reserved.