Abstract：The anti-friction of two kinds of organo-molybdenum friction modifier M1 and M2 were investigated by using cylinder-disc reciprocating mode on the SRV IV oscillating reciprocating friction and wear tester in diesel engine oil with and without diesel soot. The scanning electron microscopy/energy-dispersive X-ray detector (SEM/EDX), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS) were used to probe the mechanism that diesel soot made the organo-molybdenum friction modifier ineffective. The results indicated that diesel soot could significantly reduce the anti-friction property of organo-molybdenum friction modifier. However, M2 has better anti-friction property than M1 in diesel soot contaminated engine oil. Through the analysis of friction pairs surface by SEM/EDX and adsorption test, it demonstrates that the adsorption effect of diesel soot is the main reason which leads to ineffective of organo-molybdenum friction modifier in diesel soot contaminated engine oil.