The microstructure of nonpolar GaN layers grown on r-plane sapphire substrates was investigated using transmission electron microscopy (TEM). The structure of the GaN layers was predominantly a-plane oriented single-crystal. In the a-plane oriented layers, misoriented grains about 200 nm in size with a preferential orientation were detected by selecting a proper observation direction. The preferential orientation relationship between the a-plane oriented layers and the misoriented grains was analyzed using atomic structure models, and two possible formation mechanisms of the misoriented grains were suggested. (C) 2011 Elsevier B.V. All rights reserved.