A facile in situ replacement reaction route was successfully introduced for synthesizing Ag nanoshells with outer diameters of 40-50 nm and inner diameters of 20-30 nm using Co nanoparticles as sacrificial templates. The products were characterized by XRD, TEM, SAED, and UV-vis absorption spectra. The formation mechanism was also discussed. The reaction driving force comes from the large reduction potential gap between the Ag+/Ag and Co2+/Co redox couples, which results in the consumption of Co cores and the formation of a hollow cavity of Ag nanoshells. The UV-vis spectrum of this nanostructure exhibits a distinct difference from that of solid nanoparticles, which makes it a good candidate for application in photothermal materials.