Isotactic polypropylene (iPP) was successfully grafted onto multiwalled carbon nanotubes (MWNTs) by direct macroradical addition by sonication in hot xylene with BPO as an initiator. A quantitative determination of the grafting ratio was first realized after the iPP-g-MWNTs were extensively purified by repeated hot vacuum filtration and redispersion in xylene at 130 degrees C. It was found that both iPP macromolecular radicals and small-molecular benzoic acid free radicals were grafted onto MWNTs. iPP-g-MWNTs dispersed more uniformly in iPP than pristine MWNTs. The iPP-g-MWNTs/iPP composites have much higher elongations at break than the iPP composites filled with pristine MWNTs, and the Young's modulus and yield strength were also improved to some extent. iPP-g-MWNTs also have a stronger nucleation effect on the crystallization of iPP than pristine MWNTs.