AuPt alloy catalysts with various compositions have been successfully prepared simply by one-step co-reduction of Au and Pt precursors involving sodium citrate as stabilizer and reductant. XRD, TEM and EDX element mapping analysis confirmed that the resulting AuPt nanoparticles are single-phase alloys rather than random mixtures of tiny Au and Pt particles. Compared with Pt/C, alloying Au with Pt can effectively alter the kinetic process of formic acid oxidation, reducing the generation of CO-like intermediates. Au81Pt19 displays superior electrocatalytic activity and durability, ~11 times in the mass activity better than commercial Pt/C and may be of practical significance for the commercialization of direct formic acid fuel cell.