Processing conditions and final mechanical properties of polymer nanocomposites are affected by their interfacial layers behavior. However, it is impossible to determine directly the properties of these layers by dynamic rheometry tests. In this work, the interfacial layers properties are evaluated for polystyrene containing silica nanoparticles by the concept of glass-transition temperature shift. The samples were prepared via solution-mixing method and dynamic rheometry was used to determine the viscoelastic behavior of filled polymers in the melt state. This initial step showed that addition of silica particles increased the glass-transition temperature. By preference, decrease in the filler particle size lead to a drastic increase in the glass-transition temperature and interfacial layer volume fraction due to relatively high surface area of the small filler particles. Then, in the next step, the viscoelastic properties of interfacial layer have been evaluated on the basis of the properties of neat polystyrene using temperature-frequency superposition law. For this purpose, the shift factor was calculated from the glass-transition temperature of the sample with maximum filler content. Finally, the effect of immobilized interfacial layer on the viscoelastic properties of the polymer nanocomposite samples has been estimated. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2039-2047, 2011