We report the evolution of ZnO nanorods by metalorganic chemical vapor deposition on sapphire substrates and an investigation of their microstructure. Well-aligned ZnO nanorods with a high aspect ratio were grown on an interfacial layer with several types of defects at a lower reactor pressure. Planar defects such as stacking mismatch boundaries and inversion domain boundaries were formed in the interfacial layer during the coalescence of the islands, and finally constituted the side facets of the nanorods. Based on the microstructural changes and origin of the defects in the interfacial layers, we propose a model to explain the growth evolution of ZnO nanorods on sapphire substrates. (c) 2007 American Institute of Physics.