Transmission electron microscopy techniques are applied to investigate three-dimensional (3D) microstructures of the GaN nonplanar substrate selectively grown by metalorganic chemical vapor deposition. Two-step lateral epitaxial overgrowth (LEO) has been utilized and optimized to fabricate fully coalesced nonplanar mesa substrate templates with the trapezoidal cross-section. All threading dislocations (TDs) penetrating beyond the two adjacent mask windows are engineered to bend 90 degrees in the lower TD bending layer after the first step of growth. The dislocations, which approach the GaN mesa top, are predominantly perfect a type dislocations with Burgers vectors of 1/3 < 1120 > and a density of 8 X 10(7) cm(-2), which is reduced by three orders of magnitude compared with that of bulk GaN. The spatial distribution of different types of dislocations in the LEO nonplanar substrate is demonstrated herein. The main sources of a type dislocations in the post-bending layer are byproducts of dislocation reactions occurring at the TD, bending layer. (c) 2005 Elsevier B.V. All rights reserved.