The viscosity of motor oils of passenger cars has become lower in recent years to decrease friction resistance in the fluid lubrication region. However, this causes big problems such as higher friction and severe wear in the boundary lubrication region. So we need effectively use a friction modifier to reduce the friction and to prevent from seizure under boundary lubrication. Especially MoDTC (Molybdenum dithiocarbamate) is widely used as friction modifiers for motor oils to form lubricating film of MoS₂ to reduce friction and wear. However, it is well known that its effect is different depending on conditions such as sliding materials. For this reason, it is required to understand that reaction mechanisms in order to effectively use the additive to achieve low friction. So we tried to appear how the sliding surface changes and tribo-film of MoS₂ forms in running-in process from geometrical and chemical analysis. As a result of sliding experiment of SUJ2 against itself in MoDTC oil, formation process of low frictional tribo-film is clarified as follows; In the early stages of friction, the roughness of the self-formed friction surface is smoothed about 3 nm to 5 nm. And after formation of amorphous oxide layer on iron, MoS₂ layer is formed on it. Oxidation reaction on the friction surface during the initial adaptation process of friction is a necessary condition for tribo-film formation by MoDTC.