This paper focuses on the ZnDTP-derived tribofilm formation process in the early stage of sliding, mainly on the formation of a sulfide layer and a polyphosphate layer, in studying the formation process on a steel surface with roughness equivalent to that of an actual machine. Unlike the results of ZnDTP single-formulated oil using a regular mirror-finished steel test piece, the sulfur-based tribofilm was preferentially formed at the initial sliding stage on the steel test piece with relatively large roughness equivalent to the actual machine level. It was also found that it took longer to form a phosphorus-based tribofilm that exhibited wear resistance. Thus, the behavior of tribofilm formed on the test pieces with roughness equivalent to the actual machine level differed significantly from that of the mirror-finished test pieces. In particular, it was found that the sulfurization reaction in the early stage of sliding is important through the corrosive wear of the top of roughness, and played an important role in controlling the subsequent tribological behavior.