Highly oriented polyethylene fibers have been modified by gamma -irradiation in the presence of some vinyl monomer vapors, followed with further annealing in the atmosphere of the same monomer. Two types of vinyl monomers that are known to produce polymers with different glass transition temperatures, namely methyl methacrylate and vinyl acetate, were studied for their effect on the compressive strength of the fiber. It was found that a significant improvement in compressive strength, measured by tensile recoil test, was obtained. The level of improvement was affected by heat treatment and the presence of monomer during irradiation. Modification with vinyl acetate was found to be more effective than methyl methacrylate. These facts suggest that the improvement in compressive strength was attributable to several factors, including structural relaxation, the presence of graft copolymer, and energy dissipation ability of the graft copolymer. It is speculated that lateral integrity of the fiber is one of the key factors that prevents sliding of microfibril and possibly lateral or circumferential expansion of the fiber to accommodate kink band.