Oxygen content in a working environment affects tribological characteristics of steels. Tribological behaviors of chromium steel pins sliding against brass discs were investigated using a pin-on-disc tribometer in various environments (100%O2, 75%O2+25%N2, 50%O2+50%N2, 25%O2+75%N2 and 100%N2). The tests had performed at sliding speeds of 1 and 4 m/s and at contact pressure of 0.33 MPa. The worn surfaces and wear mechanism were analyzed by SEM, EDS and XPS. The results indicate that the wear loss of the steel pin is the lowest in 100%O2 atmosphere and highest in 100%N2 atmosphere. However, the rules of the friction coefficient and the wear are the same with the N2 content increasing in the mixed O2/N2. Furthermore, the micrographics analysis proves that the friction coefficient and the wear are strongly influenced by the surface films which depend on the tribochemistry reaction in ambient atmospheres, the adhesion between steel and brass surfaces and the mechanical properties of surface film.