Microstructures in epitaxial, lateral overgrown (ELO)-GaN were analyzed by cross-sectional transmission electron microscopy with recent to the behavior of threading dislocations and generation of horizontal dislocations (HDS). GaN was grown by two-step ELO technique controlling the facet planes. The side facets of the ELO-wing were controlled to have vertical {21 (1) over bar0} planes or slanting {21 (1) over bar2} planes. It was clarified that the mask size had an effect oil microstructures in ELO-GaN. In the case of narrow mask (window/terrace) of (3/3) mum, the c-axis of GaN was not tilted, and the area with low dislocation density was over the mask-terrace. In the case of the wide mask of (3/7) mum, HDs are generated due to the internal stress accumulated during the lateral growth and the location of HDs depended upon the direction of side facets. (C) 2002 Elsevier Science B.V. All rights reserved.