We investigate the tensile stretching behaviors of copper (Cu) nanowires (NWs) containing various vacancy defects using the embedded-atom molecular dynamics modeling approach. The coupled effects in various shapes, sizes, and locations of vacancy clusters (VC) and wire cross-sectional area on the mechanical strength and structural deformation of NWs are presented. The formation energies of VC defects are also evaluated. As the location of cluster defects changed from the edges to the surface or inside of wires, the yield strength decreased. Moreover, as the cluster shape changed from octahedron to cuboctahedron, the yield strength also substantially decreased. The extraordinarily structural evolution of defective wires that precede wire fracture from 10/4 transient multi-shell (TMS) structures necking into 1/5-type pentagonal rings were also identified via the analyses of potential energy, local stress, and atomistic number density. (c) 2010 Elsevier B.V. All rights reserved.