Nanoindentation studies have been carried out on undoped and doped epitaxial GaN thin films with different thickness (1-4 mu m) were grown epitaxially on c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Multiple discontinuities (so-called 'pop-in' events) were observed in the load-indentation depth curve irrespective of the thickness as well as the doping condition. Atomic force microscopy (AFM) studies on the residual indentation impression revealed no micro-cracks even after the indentation beyond the critical depth. The physical mechanism responsible for the 'pop-in' was explained by the interaction of the deformed region, produced by the indenter tip, with the pre-existing threading dislocation in the epitaxial GaN thin films. (c) 2005 Elsevier B.V. All rights reserved.