Pt-Mg/Al2O3 soot oxidation catalysts were prepared by impregnating either magnesium acetate or magnesium nitrate on alumina-supported platinum catalyst. The influence of Mg addition on the structure and catalytic behaviors of Pt/Al2O3 catalysts were investigated by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, transmission electron microscopy (TEM), H-2 chemisorption, thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FTIR), NOx temperature-programmed desorption (NOx-TPD), NO temperature-programmed oxidation (NO-TPO), and soot temperature-programmed oxidation (soot-TPO). In spite of the coverage of surface Pt by magnesium species and the weakened oxidation resistance of Pt, the Pt-Mg/Al2O3 catalyst derived from magnesium acetate exhibits a higher soot oxidation activity than that prepared with magnesium nitrate, which is mainly determined by the larger Pt particle size on this catalyst. Additionally, the synergistic effect between Pt and Mg enhances the NO oxidation activity and NOx storage capacity of Pt-Mg/Al2O3 catalyst. More NO2 is produced in the temperature range of soot oxidation on this catalyst than on the Mg-free Pt/Al2O3 catalyst with a similar Pt particle size, which efficiently promotes the ignition of soot.