The size-dependent thermodynamic and kinetic model was established to elucidate the physical and chemical origin of the thermodynamic driving force causing the spontaneous interfacial alloying and the anomalous diffusion rising from the metallic core-shell interface in nanoparticles. It is found that, in thermodynamic, the giant driving force from the size-dependent mixing enthalpy and interfacial energy leads to the spontaneous interfacial alloying. In kinetic, the unusually intensive interfacial diffusion provides a path to achieve the interfacial alloying at the ambient temperature. The theoretical predictions are in agreement with experimental data. (c) 2005 Elsevier B.V. All rights reserved.