We have grown group III nitride epitaxial films on Mn Zn ferrite substrates by laser MBE and investigated the surface distortion of the film using generalized grazing incidence-angle X-ray diffraction (G-GIXD), In-plane and out-of-plane diffraction spots were observed and the lattice constant of the surface region was calculated from its position. It has been found that AlN and GaN films directly grown on Mn Zn ferrite arc dilated in the perpendicular direction to the surface and compressed in the lateral direction. On the other hand, GaN grown on Mn-Zn ferrite using an AlN buffer layer is compressed in the perpendicular direction to the surface in spite of its relaxation in the lateral direction. This result indicates that the simple Hooke's law of the bulk crystal does not hold at surfaces of GaN. It can be concluded that G-GIXD is powerful technique to analyze the lattice distortion in the surface-active region of the semiconductor devices. (C) 2002 Elsevier Science B.V. All rights reserved.