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 {2110} planes or slanting {2112} planes. It was clarified that the mask size had an effect on microstructures in ELO-GaN. In the case of narrow mask (window/terrace) of (3/3)μm, 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)μm, 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.