Abstract: Experimental investigation was conducted to evaluate the effect of soot reduction by eight different organometallic compounds in a direct injection diesel engine by the free acceleration method. Meanwhile, the mechanism of these compounds was studied in the paper. All eight compounds were found to be effective in reducing soot emission with the sequence of barium dinonynalphthalene sulfonate>ferrocene>iron naphthenates>petroleum barium sulfonate>manganese naphthenates>cuprum naphthenates>barium naphthenates>cerium naphthenates. When the proportion of barium dinonynalphthalene sulfonate in the fuel is 4‰, soot emission is reduced by 40.7%, and the diesel fuel blended with 1‰ ferrocene could reduce soot emission by 35.3%. The experimental results show that the organic structure of organometallic compounds makes effect on the reduction of the soot emission, as well as on the sensitivity to the content. The formation of the soot is related to the acetylene and polycyclic aromatic hydrocarbons (PAH), while the oxidation of the soot is related to the concentration of the oxygen around and the surface area of the soot particles. Most of the organometallic compounds contain oxygen, and can break down the fuel into CO rapidly which greatly decreases the PAH. Cyclopentadienyl type organometallic compound can activate CO which prohibits the formation of the precursor of the soot.