We report a unique synthesis of core/shell Pd/FePt nanoparticles (NPs) and their catalysis of the oxygen reduction reaction (ORR). The uniform FePt shell is formed by controlled nucleation of Fe(CO)(5) in the presence of a Pt salt and Pd NPs at designated reaction temperatures. The Pd/FePt NPs show FePt shell-dependent catalytic properties, and those having a 1 nm FePt shell exhibit a drastic increase in durability and activity (15 times more active with a 140 mV gain in onset potential in comparison with those having a 3 nm coating). These Pd/FePt NPs are promising new catalysts for practical fuel cell applications.