In this study, the time-dependent formation process of molybdenum dithiocarbamate (MoDTC)-derived tribofilms at steel/steel contact under boundary lubrication was investigated by using an in situ Raman tribometer. Especially, we focused on the effects of zinc dialkyldithiophosphate (ZDDP) concentration in MoDTC solution on MoDTC tribofilm formation process. A laboratory-built in situ Raman tribometer was used to evaluate friction and the formation process of MoDTC-derived tribofilms. All our results clearly suggest that there is an optimum ZDDP concentration in MoDTC solution for promoting the formation of MoS₂ tribofilms on the sliding surfaces, and there is also a threshold value for the formation rate of MoS₂ on the sliding surface for achieving low friction under lubrication with MoDTC-containing lubricants.