We present results of first-principle studies for energetics and structural properties of CO adsorbed on the Pt-Ru surface alloys and on the surface of a bulk Pt-Ru alloy. The results showed that the Ru-CO bond strength was increased from 2.0 to 2.1 eV, whereas the Pt-CO bond was weakened from 1.54 to 1.36 eV after Ru alloying with Pt for the bulk Pt-Ru alloy. On the other hand, both Ru-CO and Pt-CO bonds are strengthened on the surface alloys.