Polypropylene (PP)/silica nanocomposites were prepared by the sol gel reaction of silicon alkoxide that was impregnated in the confined amorphous nanospace of PP with the aid of supercritical carbon dioxide. This novel technique enabled us to prepare nanocomposites having a variety of silica morphology without altering the higher-order structures of PP, being ideal to study relationships between the silica morphology and mechanical properties of the nanocomposites. The synthesized silica particles were highly dispersed in PP with dimensions comparable to the amorphous thickness (<10 nm), while their mass fractal dimension acquired by small-angle X-ray scattering was dependent on the sol gel conditions. We found that the Young's modulus as well as the storage modulus in melt viscoelastic measurements was negatively correlated with the mass fractal dimension of silica nanoparticles: A lower mass fractal dimension resulted in not only higher reinforcement but also percolation network formation at a lower silica loading. (C) 2014 Elsevier Ltd. All rights reserved.