Among existing diesel engines, fuel improvement through additives is an attractive option to meet increasingly stringent pollutant emission regulations for diesel engines. Because of the good REDOX properties of rare earth materials, it can improve the combustion performance of diesel engines and reduce pollutant emissions. This paper characterizes the performance of inorganic acid rare earth and organic acid rare earth and analyzes the emission reduction performance from the crystal structure, which aim to compare the effects of inorganic acid rare earth and organic acid rare earth on the emission reduction performance of existing diesel engines under similar conditions. The emission reduction performance of two rare earth fuel additives of cerium carbonate and cerium stearate in Changchai twin-cylinder direct injection diesel engine at five rotational speeds is contrastively studied. The experimental results show that two kinds of rare earth fuel additives can effectively reduce NO_X, CO and HC emissions of diesel engines. And the maximum reduction rate of NO_X, CO and HC in cerium carbonate is 11.37%, 14.63% and 13.80% respectively, while the maximum reduction rate of NO_X, CO and HC in cerium stearate is 10.19%, 12.15% and 9.40% respectively. Cerium stearate has higher NO_X, CO, HC and PM emissions than cerium carbonate due to its small specific surface area. So the emission reduction effect of cerium carbonate is better than that of cerium stearate.