The purpose of this work is the enhancement of the interfacial interactions between isotactic poly(propylene) and carbon nanofibers. A free radical coupling reaction was carried out by heat assisted peroxide oxidation of the nanofibers prior to their incorporation to an isotactic poly(propylene) matrix. The materials were characterized through SEM, DSC, DMTA and tensile testing. An increase of up to 17% in the Young's modulus of treated to untreated composites was attained. In combination with DSC analysis, the tensile data did not point to a possible correlation between increased stiffness and increased crystallinity. The mechanical performance of the modified composites is believed to be due to two combined opposite effects: increased chemical interaction at the interface, and peroxide‐induced isotactic poly(propylene) degradation.magnified image