Transmission electron microscopy studies were performed to reveal the underlying growth modes of ZnO nanorods by metalorganic chemical vapor deposition with respect to the type of substrates. On well-matched substrates, such as GaN/Al2O3 (0001) and ZnO (0001) substrates, a 2-dimensional (D) layer-by-layer growth occurs. During further growth after a certain critical thickness of similar to 40 nm, 1D nanorods start to grow on the 2D layer. In contrast, only vertically-aligned ZnO nanorods grow on a partially-matched substrate, such as Al2O3 (0001), after the coalescence of 3D ZnO islands. On mismatched substrates, such as Si (111) and Si (001), an Si-O amorphous layer of similar to 5 nm forms first, on which 3D ZnO islands grow. After their coalescence, 1D nanorods start to grow. (C) 2012 Elsevier B.V. All rights reserved.