Nanoindentation studies were carried out on epitaxial ZnO thin films on (0001) sapphire substrates grown by rf magnetron sputtering technique. A single sudden discontinuity ('pop-in') in the load-displacement curve was observed at a specific depth (13-16 nm) of the film. The physical mechanism responsible for the 'pop-in' event in these epitaxial films may be due to the interaction behavior of the threading dislocations during the mechanical deformation. Indentation well below the critical depth was found to be plastic deformation behavior (residual impression 4 nm). The detailed deformation behavior of ZnO thin films and the critical parameters associated with 'pop-in' phenomenon were also determined during indentation in correlation with the bulk ZnO single crystal. © 2007 Elsevier Inc. All rights reserved.