Abstract: Diesel engines can produce higher soot emissions due to its diffusion combustion mode and small amounts of fuel additives can improve such process and thereby reduce emissions. In our experiments, small amount of metal catalyst Ce was mixed with diesel fuel to investigate the effects of Ce concentration on the diesel emissions on the engine test rig. It was showed that with the increase of Ce from 5.0×10^-5 to 1.0×10^-4 in volume fraction, within the whole specified range of engine speeds, HC decreased gradually at a maximum reduction rate of 41.6%, CO decreased by 23.4% at most at low speed range, and was kept unchanged at medium-to-high speed range. However, NOx increased by 18.7% at most and soot decreased by 36.5% at low speed range and was kept unchanged at medium-to-high speed range compared with the original.