The effects of O-2 partial pressure during RF magnetron sputtering on the structural and nanomechanical properties of a-plane ZnO thin films were investigated by using X-ray diffraction (XRD) and nanoindentation techniques. The XRD and the transmission electron microscopy (TEM) selected area diffraction results indicate that the epitaxial relationship between ZnO thin films and Al2O3 substrates is ZnO (11 (2) over bar0)//Al2O3 (1 (1) over bar 02). The average values of the hardness and Young's modulus of the a-plane ZnO films were found to decrease with increasing oxygen partial pressure. The cross-sectional TEM revealing the localized plastic deformation of ZnO thin films beneath the Berkovich indenter, indicating the prominent role played by the threading dislocations in the film deformation behavior. At higher indentation loadings, the sapphire substrate exhibits extensive deformation with narrow slip bands appearing on {0001} plane. However, no evidence of pressure-induced phase transformation, as well as cracking and/or delamination phenomena at the film-substrate interface was observed. (C) 2011 Elsevier B.V. All rights reserved.