The mechanical properties of epitaxial ZnO thin films grown on (0 0 0 1) sapphire substrate were investigated by nanoindentation with a Berkovich tip and compared with that of bulk ZnO single crystal. In all indents on ZnO film a single discontinuity ('pop-in') in the load versus indentation depth data was observed at a specific depth of between 13 and 16 nm. In bulk ZnO, however only 65% of indents showed pop-in event at a specific depth of between 12 and 20 nm. The mechanism responsible for the 'pop-in' event in the epitaxial ZnO thin films as well as in bulk ZnO was attributed to the sudden propagation of dislocations, which had been pinned down by pre-existing defects, along the pyramidal {1011} and basal {0 0 0 1} planes (cross slip). The elastic modulus and hardness of the epitaxial ZnO thin films were determined to be 154 5 and 8.7 +/- 0.2 GPa, respectively, at an indentation depth of 30 nm. (c) 2005 Elsevier B.V. All rights reserved.