The influence, of environments on the properties of the surface films formed, with molybdenum dithiocarbamate (MoDTC) was examined under a reciprocating sliding condition. MoS2was formed on the rubbing surfaces in air and oxygen, and molybdenum compounds with an oxidation state lower than MoS2were produced in nitrogen and argon. The surface film composed of MoS2was effective in reducing the friction and wear, while the molybdenum compound formed in nitrogen or argon had no ability to prevent direct contact between the rubbing surfaces and to reduce the friction. It was a necessary condition for forming the surface film composed of MoS2that the environment to be rubbed contained oxygen at a concentration above a certain level.