A new type of core-shell nanostructure consisting of Au nanoparticles embedded in shells of Ag nanoparticles-filled polymer has been fabricated in a two-step procedure, in which Ag nanoparticles served as initial reductive agent for Au3+ ions with the existence of poly(vinylpyrrolidone). Upon ultrasonic irradiation, solidification and spherulite growth of the polymer was induced around the as-formed Au nanoparticles such that very small Ag nanoparticles were embedded in the crystalline polymer spherulites. UV/vis optical spectroscopy, transmission electron microscopy and high-resolution transmission electron microscopy have been used for structural characterization of these materials at various stages of the sonochemical synthesis. The growth mechanism was proposed for the composite nanostructure of Au-core/Ag-PVPshell. (C) 2008 Elsevier B.V. All rights reserved.