On the basis of thin iflm lubrication theory, the inlfuence of lfuid iflm (disordered iflm), ordered iflm and ad-sorbed iflm on tribological behavior of lubricating oil in thin-iflm lubrication (TFL) regime was studied. Theμ-L (friction coefifcient versus load) curves of different oil viscosity and additive dosage were obtained by a high frequency reciprocat-ing test rig and the adsorption capacity of additive on steel surface were measured by QCM-D. Based on the Stribeck curve and thin iflm lubrication theory model, some conclusions can be drawn up, namely:(1) Theμ-L curves and the parameters of L0 andμ0, obtained from the high frequency reciprocating test rig with ball-disc contact, can be used to study tribologi-cal behaviors of lubricating oil under TFL conditions. (2) In comparison with the high viscosity base lfuid, the lower one can enter into TFL regime under lower load and keeps a lower friction coefifcient in TFL regime. (3) The polar molecules in additive formulation produce ordered adsorbed layer on steel surface to reduce friction coefifcient. And in TFL regime, the molecule’s polarity, layer thickness and saturation degree on steel surface probably can inlfuence lubricant’s tribological behaviors between the moving interfaces. Moreover, the further study would be focused on the competitive adsorption of different additives, the formation of dual-and/or tri-molecular adsorption layers, and other aspects.