In this study, the synergistic elfects of ethylene-propane and ethylene-dimethyl ester (DME) mirtures on soot formation were investigated experimentully using u coflow dilfusion flame burnen The soot volume fraction, soot particle diameter and number density were measured and compured to the homogenous mixture. Addition of DME and propane to the ethylene fuel increased soot volume fraction in the ethylene flames. The ethylene- propnne has more pronounced synergistic ffict in compsrison to the ethylene-DME flame* This is due to the fact that during the decomposition of propane some methyl radicals are generated, The reuctions related to these methyl radicals promote the formation of propargyl rodiculs and conseqaently the formation of benzene through propargyl self-reaction and finally to the soot formation. Althoagh DME decomposition produces methyl, the C-O bond in the DME removes some carbon from the reaction puth that produces soot, Hence the soot formation in ethylene-DME mixture is much slower than that in ethylene-propane mixtuFe,