The aim of this article is improved the surface properties of Poly[p-phenylenebenzobisoxazole] (PBO) fiber with epichlorohydrin hybridized carboxylic multi walled carbon nanotubes (MWCNTs-Ecp) grafting by using c-ray irradiation technology. The surface chemical properties, the surface morphology, the amount of the grafted MWCNTs on PBO fiber and the surface free energy of PBO fibers have been analyzed. The results show that MWCNTs-Ecp have been grafted on the surface of PBO fiber by gamma-ray irradiation treatment. The surface chemical inertness and the surface smoothness of PBO fiber are significantly improved by grafting MWCNTs-Ecp chains, the amount of the grafted MWCNTs on PBO fiber is about 11.9%, and the surface free energy of PBO fiber has an increase of 42.6% by generating some active groups such as -COOH, -OH, and -C-Cl on the surface of PBO fiber. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3455-3459, 2011