The boundary lubrication performance of organomolybdenum compound under the condition using with phosphate ester was studied, and the lower friction mechanism in the condition is discussed in this paper. Ball on plate type spinning friction test was conducted, and molybdenum dithiocarbamate (MoDTC) and phosphate ester (tricresyl phosphate, TCP) were used as additives in the test. Results show that the friction coefficient after the test in the condition using MoDTC and TCP was lower than that in the condition using only MoDTC, and the time with lower friction coefficient in MoDTC and TCP was longer than that in MoDTC. It is also shown from energy dispersive X-ray spectroscopy analysis results after the test in MoDTC and TCP that the chemical reaction film formed on the friction surface consists of phosphorus, oxygen and sulfur, and it should be layer film of iron phosphate and molybdenum disulfide. These results suggest that the chemical reaction film formed by TCP should offer the effective action for the formation of molybdenum disulfide and the prevention of contact between iron surfaces.