c-axis oriented ZnO thin films are epitaxially grown on MgO (001) substrates by pulsed laser deposition. The orientation relations between the films and the substrates and the atomic details at the interfaces are investigated by means of conventional and aberration-corrected transmission electron microscopy. The ZnO thin films show a microstructure of columnar grains that follow the two types of in-plane orientation relations with the MgO substrates: [11 (2) over bar0](ZnO)//[1 (1) over bar0](MgO) and [1 (1) over bar 00](ZnO)//[1 (1) over bar0](MgO). These columnar grains grow with either an oxygen atom plane or a zinc atom plane as the starting plane on the charge-neutral MgO (001) surfaces, forming locally polar down or polar up interface, respectively. The relative positions of different atoms including oxygen at the interface are determined. (C) 2014 Elsevier B. V. All rights reserved.