Ni@Ag core-shell nanoparticles supported on SiO2 were prepared by a galvanic replacement reaction between Ni and Ag at a low temperature (213 K). The Ni@Ag core-shell catalyst showed a higher catalytic activity for CO oxidation than Ag and Ni monometallic catalysts as well as than Ni-Ag bimetallic catalyst prepared by a conventional impregnation method.