CO oxidation at low temperature over Pd/CeO2-TiO2 catalyst was carried out in the feed containing different contents of water vapor (H2O). A positive effect of H2O was observed on the catalytic performance of Pd/CeO2-TiO2 in CO oxidation at low temperature. The extent of this effect depends on the content of H2O in the feed; with a H2O content being 2.5 vol%, the catalyst Pd/CeO2-TiO2 exhibits the highest stability (longest life time for CO oxidation at 80 C). The results of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed reaction (TPReaction) reaction illustrated that H2O in the feed supplies sufficient OH groups in the presence of O-2, which can react with adsorbed CO on Pd species to produce CO2. Moreover, H2O may also enhance the adsorption of CO and suppress the formation of some carbonate species.