Abstract: By Way of SEM observation for the carbon fiber surface and the determination of the mechanical properties of conjugate fibers,it is found that plasma treatment almost has no affection on the modulus of the carbon fiber,on the other hand, the fiber strength drops slightly with the increasing of the plasma power or with the extending of the treating time.only ex-pression in outward appearance of the fiber is the dapening and the broadsning of the ditches on the fiber surface,along the fiber axle. Furthermore. the injury to carbon fiber of Ar plasma treatment is not as serious as that of air plasma treatment,which has oxidizing property. secondly,by combining Raman spectra with XPS analyses,the effective depth of plasma treat-mant is found to be about 15 nm. After plasma treatment,short aliphatic molecule chains or functional groups on the surface of the carbon fiber are etched off and the structure of graphite crystal is concentrated on the surface with high potential.