Polypropylene (PP)/C60 nanocomposites with improved interfacial interaction were prepared via in situ melt radical reaction. It was found that the relaxation time of PP/C60 nanocomposites containing peroxide increased due to the reaction between C60 and PP macroradicals and the formation of long chain branched or crosslinking structure. Thermogravimetric analysis (TGA) results showed that the thermal stability of PP/C60 nanocomposites was enhanced. The initial decomposition temperatures and activation energy of PP/C60 nancomposites were strongly influenced by the content of unreacted C60. Because of the improved interfacial interaction, PP/C60 nanocomposites containing peroxide showed obvious increase in tensile strength, Young' modulus, flexural strength, flexural modulus, and impact strength, compared to PP/C60 nanocomposites (without peroxide) containing the same content of C60. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers