Mn-Pt nanocubes were synthesized from platinum acetylacetonate and manganese carbonyl in the presence of oleic acid and oleylamine. The Mn-Pt nanocubes were converted into an ordered MnPt3 intermetallic phase upon annealing. The electrocatalytic properties of the Mn-Pt nanocubes and spheres (including oxygen reduction, formic acid oxidation, and methanol oxidation) were tested and found to be shape-dependent. The Mn-Pt nanocubes, which are enclosed by (100) surfaces, showed better activities than their spherical counterparts. These materials are promising new candidates as cathode and anode catalysts in fuel cells.