Polypropylene (PP) composites containing nanosized (phi similar to 10 nm) spherical silica particles were prepared in situ utilizing a 1-L slurry-phase polymerization reactor containing a MgCl2-supported Ziegler-Natta (fourth-generation) catalyst. Composites were prepared with variable filler sizes ranging from the nano- to microsize domain. The surface of the silica particles was modified with a silane coupling agent to prevent catalyst deactivation and to achieve better polymer/ filler synergy by decreasing the hydrophobicity surrounding the bulk particle surface. The feasibility of the process was tested with low loading additions (similar to3 wt % for each sample) of (nano)silica. The effect of the filler on the crystal structure and crystallite size of PP was studied with differential scanning calorimetry and wide-angle X-ray diffraction. The reaction kinetics of the nanocomposite (similar to10 nm) was studied and compared with samples containing larger microsized particles. It was demonstrated that the former did not decrease the catalyst activity when compared with the latter; comparisons were also made with pure PP. The particle size influenced the reactivity of the catalyst, and the addition of silica influenced the degree of crystallinity of the PP.