Abstract: Abstract: Nitrogen oxide (NOx) and particulate matter (PM) are the main emissions for diesel engines. Because of their contradictory relationship of generation mechanisms, only using the internal purification technology is very difficult to meet the increasingly stringent diesel emissions regulations. Development and application of after-treatment technology with low cost, high efficiency and high adaptability will be more promising, which should be utilized to control both NOx and PM emissions. Rare-earth-based catalysts have rich electronic structure, and show the unique physical and chemical properties. In existing rare earth oxides, cerium oxide has been paid much attention in the field of catalysis because of its low price, unique crystal structure and reversible transformation of trivalent ion (Ce3+) and tetravalent ion (Ce4+). In the recent years, the application of cerium dioxide (CeO2) in after-treatment technology for diesel engine is a hot research topic. In this study, 3 groups of nano-CeO2 were prepared using the coprecipitation method in order to reduce the PM and NOx emissions from diesel engine through the after-treatment technology. The samples were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and hydrogen temperature programmed reduction (H2-TPR). What was more, the activity of catalysts was evaluated by ignition temperature and peak temperature of soot combustion as well as conversion ratio from nitric oxide (NO) to nitrogen (N2). The experimental results showed that CeO2 crystal structure had not been changed, and continued to be the cubic fluorite structure. The average particle diameters of the prepared CeO2 were 7, 12 and 20 nm, respectively, which were much smaller than that of commercial CeO2. Compared with commercial CeO2, the prepared CeO2 had larger specific surface area, which indicated that there were more active sites on the surface of CeO2 for the unit mass. Furthermore, there were more opportunities for the catalyst to be exposed to the reactants, which was beneficial for adsorption and activation of the reactant molecules. The prepared CeO2 had 3 obvious H2 reduction peaks, corresponding to the surface absorbed oxygen, surface lattice oxygen and bulk lattice oxygen, respectively. Oxygen species, especially the surface lattice oxygen, had direct relation with catalytic activity. The reduction property of surface oxygen species was stronger, and the catalytic activity was higher. The results of H2-TPR had correspondence with the results of BET. For the efficiency of catalytic oxidation, the order of nano-CeO2 particle size from high to low was 20, 12 and 7 nm, successively. The ignition temperatures of soot combustion were reduced by 124, 109 and 93 ℃, and the peak temperatures were reduced by 185, 104 and 102 ℃ respectively with the 3 groups of CeO2 catalysts. With the increase of temperature, the conversion ratio of NO firstly increased and then decreased. The conversion ratio of NO with 20 nm CeO2 reached the highest value of 70% at 350 ℃. The conversion ratio of the 3 groups of CeO2 catalysts was higher than 68% at 400-520 ℃, which indicated that CeO2 has a wide temperature window. The experimental results can provide a reference for optimum design and application of CeO2 catalyst in the field of diesel exhaust after-treatment system.