As CO poisoning of platinum is one of the most important problems in PEFC, many studies are conducted to find alternative catalysis but replacing Platinum with alternative materials seems to be still difficult. In this study, another approach to find a scavenger that enhances CO removal is proposed. To this objective, behaviors of adsorbed CO on Pt surface are studied using Density Functional Theory (DFT). The results show that effect of a surrounding H_2O molecule on CO-Pt adsorption energy is small and the energy barrier of the reaction of ad-CO and O is about 0.4 eV and CO_2 produced can easily leave Pt surface.