PP/TiO2 nanocomposites were prepared from an original method based on the hydrolysis-condensation (sal-gel method) reactions of titanium alkoxide inorganic precursor premixed with polypropylene (PP) under molten conditions. Nanocomposites with a mean diameter of primary particles lower than 5 nm were then prepared. The TiO2 particle dispersion in the PP matrix was characterized over a wide length scale from the combination of small angle X-ray scattering, transmission electron microscopy, and linear viscoelasticty of molten nanocomposites. As a result, a fractal structure of these particles was highlighted at the highest concentration (phi(r) >= 0.014) with a characteristic aggregation size d(aggr) approximate to 130 nm. The relationships between fractal structure and linear viscoelastic have been discussed from the main works of the literature on the reinforcement of nanocomposites. The drastic alteration of the terminal relaxation zone (solid-like behavior) is correlated to the formation of an aggregate-particle network. The study of the nonlinear viscoelastic behavior (Payne effect) agrees qualitatively with this reinforcement mechanism. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1213-1222, 2010