The deformation behavior of electrolessly deposited nanocrystalline copper films with an ultrafine grain size of about 10 nm has been investigated in this study by using instrumented nanoindentation. The hardness, shear stress for plastic yielding, and energy release rate of the electroless copper films were obviously lower than those of electroplated copper films with a larger grain size due to the dominant deformation mechanism of grain-boundary sliding and grain rotation instead of dislocation activity. Voiding at grain boundaries and triple grain junctions was observed in the indented region as a consequence of grain boundary sliding and grain rotation. (c) 2006 The Electrochemical Society.