Aiming to develop a high performance fiber-reinforced natural rubber (NR), a special technique using electron beam (EB) irradiation-induced graft-polymerization was applied to high-strength polyethylene terephthalate (PET) fibers. Although PET is chemically robust, allyl methacrylate (AMA) could be graft-polymerized onto the PET fiber surface with this special technique. The composite of NR and grafted-PET fibers indicated a linear increase in the initial modulus with the fiber content. At the fiber content of 10%, the initial modulus was improved about five times with respect to that of the pure NR, whereas the breaking strength increased by ca. 35% and the breaking strain decreased by ca. 45%. The fiber-reinforced rubber with a good performance was obtained in the system of NR and grafted-PET fibers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2584-2590, 2009