Abstract: The wicking method is used for investigating the effect of plasma treatment onthe wettability of polyacrylonitrile-based carbon fibers.It is found that the wettability ofthe fiber can be improved continuously with increasing plasma power or extending treatmenttime and improved more greatly with O_2 plasma treatment than with Ar plasma treatment.However,there exists a limit of treating power or treating time. When exceeding the limit.the wettability of the fiber will go bad instead.Furthermore,the graft-polymerization methodthat the maleic anhydride is coated on to the surface of the carbon fiber which is treated withO_2 plasma later,is verlfied.It can be seen that the wettability of the grafted caibon fiber isnot better than of urltreated fiber at last,it is made clear that after plasma trcatment,the wett-ability of the fiber surface wiil deeay gradually with the time for the fiber existing in air.The contact angle of fiber with water increases zigzag after O_2 plasma treatment.