The mutual transposition of the core and the shell of a Au@Pt core-shell nanosphere has been obtained by employing picosecond laser pulses to excite the surface-plasmon resonances of platinum. The thermalized energy of the plasmon resonances makes the core metal of the gold melt earlier than the shell metal of platinum because of melting temperature differences and causes the gold to soak out of the core to the surface of the nanosphere. A new reversed core/shell Pt@Au core-shell nanosphere is formed with further irradiation.