Zinc dialkyldithiophosphate (ZDDP) improves wear resistance by forming a tribo-film on the sliding surface. However, the tribo-film is known to cause an increase in the friction coefficient. In order to reduce the friction coefficient as well as the wear, it is expected to combine an organic friction modifier (OFM) with ZDDP. In this study, a macro-friction test was conducted using a reciprocating sliding tester under the lubrication with the combined oils of ZDDP and OFMs. In order to understand the lubrication mechanism of the combined oils, the formation process of tribo-film was evaluated by using AFM in-situ observation. NMR analysis of the combined oil and XPS analysis of the sliding surface were performed. Glycerol monooleate (GMO) was found to suppress the formation of ZDDP-derived tribo-film. On the other hand, it was found that tallow diethanolamine (TDEA) changes the composition and mechanical properties of the ZDDP-derived tribo-film and exerts a friction-reducing effect. It was considered that the combined effect of OFM is due to the competitive adsorption phenomenon and the change of ZDDP molecular structure in the mixed oil.