Flower-shaped Pt-Pd nanostructures supported on carbon (Pt-Pd/C) were successfully synthesized by simple wet-chemical processing by controlling the rate of reduction of a dissolved metal precursor. L-ascorbic acid (AA) was employed as a green and clean reductive agent for the coreduction of [PtCl6](2-) and [PdCl4](2-) in an aqueous solution. The electrocatalytic activity of nanoporous Pt-Pd/C on the oxygen reduction reaction (ORR) was investigated by rotating disk electrode voltammetry in a 0.1 M HClO4 solution. In particular, the Pt-77-Pd-23/C catalyst showed surpassing ORR activity and acceptable stability under acidic conditions compared to the commercial Pt electrocatalyst. The outstanding electrocatalytic activity was achieved by a combination of the high surface area of flower-like nanostructures and the bimetallic synergetic effect. The morphology, composition and structure of the Pt-Pd/C nanoflowers were confirmed by scanning transmission electron microscopy (STEM), TEM, X-ray photoelectron spectroscopy and X-ray diffraction. Overall, the Pt-Pd/C bimetallic nanostructures are expected to become promising cathode material in fuel cells to replace monometallic Pt ones.