For improving durability of hydraulic parts as for construction machine, it is considered that understanding the details of tribofilms formation derived from additives in lubricating oil, as well as applying of surface treatment such as nitriding to conventional steel, is important. During operation, these parts are usually lubricated with fully formulated oils, which include various additives such as dialkyldithiophosphate (ZnDTP), dispersant, detergent, etc. In this study, the tribological features of two kinds of steels lubricated with fully formulated oil and sample oils were evaluated by changing the combination of additive formulations, and the effect of additive formulation on tribofilms formation, frictional and wear property were investigated in detail. As a result, it was revealed that there is a large interaction between the each surface of steels and additive, especially, coexistence of ZnDTP and detergent affects the phosphorous-derived tribofilms formation as well as their frictional or wear properties.