Classical molecular dynamics simulations have been employed to estimate the energy required to form vacancy in unsupported nanometre-sized spherical particles of Au and Cu. Their dynamics has been also investigated to corroborate the vacancy formation energy estimates and to characterize its relationship with particle size. It is shown that the particle is a strongly inhomogeneous system. The generation of vacancies at random in its interior permits to identify three different regions characterized by as much different values of vacancy formation energy. The bulk-like region exhibits values equal to the ones pertaining to bulk systems, whereas the vacancy formation energy increases as the surface layer is approached. It follows that the vacancy formation energy is slightly higher in nanometre-sized particles than in bulk. (C) 2008 Elsevier BY. All rights reserved.