An approach to rationally design optimal alloy catalysts is established using nitric oxide (NO) oxidation as an example. We introduce a quantitative structure-energy equation to predict the chemisorption energies of adsorbates on alloy catalysts. The structure-energy descriptor is used to rationally design Pt-based and Ni-based alloy catalysts for NO oxidation. Full first principles calculations with kinetic simulations demonstrate that these designed catalysts possess much better catalytic performances than the traditional catalysts.