Adsorption mechanism of carbon monoxide (CO) on PtRu and PtRuMo alloy surfaces is investigated using density functional theory (DFT). It includes evaluation of binding configuration and the adsorption strength. The results show that CO preferentially adsorbs onto the 3 fold hollow site of the PtRu-surface, while on the PtRuMo surface we observe the shift from the fcc hollow site to near the bridge site. We also note that adsorption energy of CO on the PtRuMo is stronger than that of adsorption on the PtRu surface. From the charge transfer analysis, we conclude that the stronger binding energy is caused by the more charge transferred to the surface-adsorbate bonding region brought by alloying Mo to the PtRu.