A series of carbon supported palladium copper alloyed catalysts (PdCu/C) were prepared using different molar ratios of Cu to Pd (Cu/Pd), namely, Cu/Pd=1.0, 2.0, 3.0, 4.0, 4.5, and 5.0, by a facile two-step method. The Cu nanoparticles were obtained by microwave assisted route, and followed by the formation of PdCu/C nanoparticles through galvanic substitution of Pd on Cu nanoparticles. The effects of Cu/Pd ratio on the microstructure and electrocatalytic activity of PdCu/C toward oxygen reduction reaction (ORR) were systematically investigated. The as-prepared PdCu nanoparticles exhibited mainly cubic phase with average crystallite sizes ranging 4.7~6.7 nm by varying the Cu/Pd ratio of 1.0~5.0. The higher alloying degree and better ORR activity could be achieved at Cu/Pd=4.5. The half-wave potential (E_1/2) reached 0.760 V in 0.1 mol/L HClO₄ solution, which was 40 mV lower than that of commercial Pt/C, with a four-electron transfer pathway toward ORR.