Zinc oxide films were deposited on (0001) and r-cut alpha-Al2O3 under identical conditions using metal organic chemical vapor deposition. Microstructures of the ZnO films were studied in detail using conventional and high-resolution transmission electron microscopy (HRTEM), electron diffraction, and HRTEM image simulations. The films deposited on these two substrates show distinctive structural differences. The Film grown on r-cut alpha-Al2O3 shows a high-quality single crystal with an orientation relationship of alpha-Al2O3[-101-1]//ZnO[0001] and (alpha-Al2O3(10-1-2)//ZnO(2-1-10). The interface between the film and the substrate was abrupt and decorated with high density of misfit dislocations. Film grown on alpha-Al2O3 (0001) shows several orientation domains. Typically, one domain correspond to the classic growth model such that alpha-Al2O3 (0001)//ZnO(0001) and alpha-Al2O3 [11-20]//ZnO[10-10]. Another domain corresponds to the growth mode such that alpha-Al2O3 [11-20]//ZnO[10-10] but the (0001) plane of ZnO is tilted relative to the (0001) plane of alpha-Al2O3 such that ZnO(0001) is almost parallel to the alpha-Al2O3 (-1104) plane. This orientation reduces the extent of lattice mismatch as compared with the classic growth mode. The interface between ZnO and alpha-Al2O3 is abrupt and possesses periodic dislocations. (C) 2008 Elsevier B.V. All rights reserved.