Composites of polypropylene (PP) with nontreated nanosilica at different particle sizes are prepared, and their viscoelastic properties are studied by dynamic strain- and frequency-sweep experiments. It is clear that incorporation of nanoparticles in a polymer matrix due to the huge specific surface area enhances both moduli so that the storage modulus is intensified more than loss modulus. The specific surface area of a particles increases as a particle size decreases, leading to a stronger tendency to form reinforcing structures. In nanocomposites of smaller particles due to existence of a larger contact area between particles, more and stronger filler clusters are formed. Thus, a stronger filler filler network is formed by decreasing the particle size so that both storage and loss moduli present higher values. The slope of dynamic moduli as a function of frequency decreases with filler loading augmentation in a low-frequency region, and this solid-like behavior becomes more significant with particle size reduction. The PP composites in high filler loadings exhibit a yield stress, and weight fraction corresponds to the onset of this well-defined yield stress and is lower in the case of smaller filler particles. In addition, the polymer chains adsorption on the particles and/or clusters surface and formation of filler polymer network increases with filler loading augmentation. The influence of these two reinforcing mechanisms leads to a gradual enhancement of the Payne effect and the solid-like behavior in strain-and frequency-sweep experiments, respectively. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 203-218,2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20217