Nano-sized calcium carbonate was pretreated with silane coupling agent and then mixed with butyl acrylate that is of larger amount than the nanoparticles. Under gamma-irradiation, graft polymerization occurred on the nanoparticle surface, forming a nanocomposite structure consisting of grafted poly(butyl acrylate) (PBA), homopolymerized PBA, and the segregated nanoparticles. It was found that the silane pretreatment significantly promoted the graft reaction. When the grafted nano-CaCO3 particles were melt compounded with polypropylene (PP), an obvious synergistic effect, offered by (i) the chemical bonding between the elastomer type grafted PBA and nano-CaCO3 and (ii) the deliberately introduced thick interlayer mainly constructed by the homopolymerized PBA, led to a significant increase in notch impact strengths and elongation to break of PP at a rather low content of nano-CaCO3. Meanwhile, the tensile stiffness of the composites was also slightly increased and the yielding strength of the composites was almost unchanged. The results are different from those with conventional rubber-toughened plastics, in which the improvement of ductility is acquired at high additive fraction and a great expense of strength performance. (c) 2005 Society of Plastics Engineers.