We report a new seed-mediated growth of FePt over 2.5 nm wide FePtM (M = Pd, Au) nanowires (NWs) into core/shell FePtM/FePt NWs with controlled FePt shell thickness from 0.3 to 1.3 nm. These FePtM/FePt NWs show shell thickness and core composition-dependent electrocatalytic activity for oxygen reduction reaction (ORR) in 0.1 M HClO4. These core/shell FePtM/FePt NWs are generally more active and durable than the corresponding alloy NWs. Among FePtM/FePt NWs, FePt NWs, FePtPd NWs, and commercial Pt studied, the FePtPd/FePt NWs (0.8 nm shell) show the specific activity of 3.47 mA·cm(-2) and the mass activity of 1.68 A/mg Pt at 0.5 V (vs. Ag/AgCl), superior to all other NWs (less than 1.59 mA/cm(2) and 0.82 A/mg Pt for FePtAu/FePt and FePt) as well as the commercial Pt (0.24 mA/cm(2) and 0.141 A/mg Pt). The FePtM/FePt (0.8 nm shell) NWs are also stable in the ORR condition and show no activity decrease after 5000 potential sweeps between 0.4 and 0.8 V (vs Ag/AgCl). They are the most efficient nanocatalyst ever reported for ORR.