This article presents the investigation on the epitaxial growth of large scale spatially ordered ZnO nanostructures on (0001) c- and (11 (2) over bar0) a-plane sapphire substrates by seed-assisted hydrothermal growth without the need for any complicated lithography or pre-patterned catalyst. Prior to hydrothermal growth of ZnO nanostructures, ZnO seeds were epitaxially grown on as-annealed sapphire substrates by metal organic chemical vapor deposition (MOCVD). The periodic ZnO nanorod rows with a fairly constant separation of 100 nm were directly grown on the c-plane sapphire substrates. However, the linear periodic array of ZnO nanowalls with the thickness of about 240 nm was formed on the a-plane sapphire substrate by coalescence of numerous vertically aligned ZnO nanorods. The average spacing between the ZnO nanowalls is measured to be 450 nm. The microstructural characterization of the self-ordered ZnO nanowall array was further performed by cross-sectional transmission electron microscopy. Sapphire surface structure and CVD growth kinetics closely relate to the self-organized epitaxial growth of ZnO nanostructures on c- or a-plane sapphire substrates. (C) 2012 Elsevier B.V. All rights reserved.