In this study, we investigated different palladiumsupport interactions over the Pd/CeO2 and Pd/CeO2 ZrO2 catalyst and illuminated the effect of support on CO oxidation performance. Pd/CeO2, Pd/Ceo.7Zr0.3O2, and Pd/Ce(0.3)Zr(0.7)0O were prepared by impregnation, and their structural properties and palladium states were characterized systematically. Results demonstrate that the interaction is reflected as the strength of palladium-oxygen bonding, which depends on the amount of surface oxygen of supports. For this reason, palladium species prefers to exist as the ionic state on Pd/CeO2 but as the metallic state on Pd/Ce(0.3)Zr(0.7)0O(.) Although Pd/CeO2 exhibits higher palladium dispersion, it presents lower TOFs for CO oxidation because of the weak CO adsorption capacity of Pc12+. After hydrothermal aging, palladium species interacts less with the support, resulting in the presence of metallic Pd and bulk PdO. The aged catalysts show the same TOFs for CO oxidation. Aged Pd/Ce0.7Zr0.3O2 exhibits higher palladium dispersion and superior activity.