Pd-based catalysts modified by cobalt were prepared by co-impregnation and sequential impregnation methods,and characterized by X-ray powder diffraction (XRD),N2 adsorption/desorption (Brunauer-Emmet-Teller method),CO-chemisorption and X-ray photoelectron spectroscopy (XPS).The activity of Pd catalysts was tested in the simulated exhaust gas from lean-burn natural gas vehicles.The effect of Co on the performance of water poisoning resistance for Pd catalysts was estimated in the simulated exhaust gas with and without the presence of water vapor.It was found that the effect of Co significantly depended on the preparation process.PdCo/La-Al2O3 catalyst prepared by co-impregnation exhibited better water-resistant performance.The results of XPS indicated that both CoAl2O4 and Co3O4 were present in the Pd catalysts modified by Co.For the catalyst prepared by sequential impregnation method,the ratio of CoAl2O4/Co3O4 was higher than that of the catalyst prepared by co-impregnation method.It could be concluded that Co3O4 played an important role in improving water-resistant performance. Pd-based catalysts modified by cobalt were prepared by co-impregnation and sequential impregnation methods, and characterized by X-ray powder diffraction (XRD), N2 adsorption / desorption (Brunauer-Emmet-Teller method) photoelectron spectroscopy (XPS). The activity of Pd catalysts was tested in the simulated exhaust gas from lean-burn natural gas vehicles. The effect of Co on the performance of water poisoning resistance for Pd catalysts was estimated in the simulated exhaust gas with and without the presence of water vapor. It was found that the effect of Co significantly depended on the preparation process. PdCo / La-Al2O3 catalyst prepared by co-impregnation successfully better water-resistant performance. The results of XPS indicated that both CoAl2O4 and Co3O4 were present in the Pd catalysts modified by Co. For the catalyst prepared by sequential impregnation method, the ratio of CoAl2O4 / Co3O4 was higher than that of the catalyst prepared by co-impregnation method. It c ould be meant that Co3O4 played an important role in improving water-resistant performance.