Assemblies of gold(core)/copper(shell) nanoparticles could be prepared through the electrochemical deposition of copper metal onto self-assembled gold colloids that are immobilized via an organic film onto conductive substrates. In principle, the deposition on each particle is required to be independent of those on other particles, to develop monodispersed core-shell particles. From a simplified model, we derive formulas for the criteria to ensure the deposition independence, demonstrating the required experimental conditions of particle size, assembly density, and the ratio of the surface concentration to the bulk concentration of depositing species. In the electrolyte of 1.0 mM CuSO4/0.10 M Na2SO4, we then manage to realize the experimental conditions lying in the independent region through applying a potential of -0.030 V(vs SCE) onto self-assembled gold particles 13 +/- 1.0 nm in size, with a density of 350 mum(-2). The resulting core-shell nanoparticles show high monodispersity, especially at a relatively smaller size, because the particle size is <20 nm (with a relative standard deviation of RSD < 10%).