A grid of regularly-distributed misfit dislocation (MD) arrays is observed in GaN films grown on AlN buffer layers by plan-view and cross-sectional transmission electron microscopy (TEM). It is found that the spacing of the MD arrays decreases with increasing the GaN film thickness. Before the formation of grids in very thin GaN films, short MDs originated around and bordered on long dislocations probably in order to achieve an isotropic strain relaxation, which indicate that the MD grid is formed by a progressive introduction of MDs with the GaN growth. Furthermore, high resolution TEM displays that, due to large misfit strain between GaN and AlN, a part of the MDs is produced by interfacial migration of pre-existing threading dislocations (TDs) in AlN layers at the early stage of the GaN growth. The MDs in GaN are thus proposed to be introduced first by interfacial migration of TDs in AlN and then gradually form into a grid around the misfit segments of TDs. This is supported by a good agreement between experimental stain values of the GaN films and a theoretical prediction based on the TD migration. (C) 2008 Elsevier B.V. All rights reserved.