We report a facile synthesis of Pt-Pd bimetallic nanoparticles, named "metallic nanocages", with a hollow interior and porous dendritic shell. This synthesis is easily achieved by selective chemical etching of Pd cores from dendritic Pt-on-Pd nanoparticles. The obtained Pt-Pd nanocages show superior catalytic activity for methanol oxidation reaction compared to other Pt-based materials reported previously.