Ultrathin metal layer-coated particles have potential applications in various fields, especially in electrocatalysis, where catalytic activity can be increased by shell@core design. In this article, a synthetic method is introduced to synthesize shell@core nanoparticles, in which the selected reducing agent can be electrochemically oxidized preferentially on the core particle but not on the shell metal. Once the shell metal is deposited on the core metal, the oxidation of the reducing agent is inhibited, subsequently forming a shell@core particle with an ultrathin shell. By this method, carbon-supported shell­(Pt)@core­(Pd) nanoparticles with submonolayer Pt shell were synthesized using formic acid as reducing agent. Spectroscopic characterizations, X-ray photoelectron spectroscopy and energy-dispersive system, confirmed the Pt deposition. The shell@core structure of Pt@Pd was corroborated by scanning transmission electron microscopy analysis. This Pt­(shell)@Pd­(core) electrocatalyst was tested for electrochemical reduction of oxygen. Further, the influence of lattice parameter on the catalytic activity for oxygen reduction reaction was examined by varying the lattice parameter of Pt@Pd nanoparticles.