Carbon fibers are expected to low friction materials. By knitting carbon fibers, it is assumed to be easier to manufacture low friction area rather than several carbonaceous coating such as DLC. However, friction coefficient of carbon fibers is not clarified and it is required to obtain low friction surface of carbon fibers with some surface modification technology. Ar-N_2 plasma irradiation was utilized to reduce the friction coefficient of carbon fibers. Static friction coefficient of carbon fibers was approximately 0.4 against SUJ2, Si_3N_4 ,DLC and carbon fibers. They depended on normal load. On the other hand, kinetic friction coefficient of carbon fibers was approximately 0.2 against all of mating materials. They did not depend on normal load so much. After the 1 0 minute irradiation of Ar-N_2 plasma , static and kinetic friction coefficients of carbon fibers decreased from 0.4 to 0.3. Kinetic friction coefficient of Ar-N_2 plasma irradiated carbon fiber decreased from 0.2 to 0.15.