Nanoindentation studies are carried out on epitaxial ZnO and GaN thin films on (0 0 0 1) sapphire and silicon substrates, respectively. A single discontinuity ('pop-in') in the load-indentation depth curve is observed for ZnO and GaN films at a specific depths of 13-16 and 23-26 nm, respectively. The physical mechanism responsible for the 'pop-in' event in these epitaxial films may be due to the interaction behavior of the indenter tip with the pre-existing threading dislocations present in the films during mechanical deformation. It is observed that the 'pop-in' depth is dependent on lattice mismatch of the epitaxial thin film with the substrate, the higher the lattice mismatch the shallower the critical 'pop-in' depth. © 2007 Elsevier B.V. All rights reserved.