Nanocomposites of acrylonitrilebutadienestyrene (ABS) and nanosilica with different nanoparticle sizes and various loadings are prepared. Rheological experiments such as frequency sweep, strain sweep, and rotational test are performed to investigate the influence of nanoparticle loading and size on the viscoelastic properties of the nanocomposites. The results show that nanocomposites with higher filler loading and smaller particle size have both higher storage and loss moduli. Moreover, the results indicate that the storage modulus is more sensitive than loss modulus to filler loading and nanoparticle size. The smaller nanoparticles and higher filler loadings lead to the enhancement of nanoparticle surface area so that the viscoelastic properties are intensified through increase of polymer chain adsorption on nanoparticle, and creation of a network structure in the nanocomposites. The network structure causes changes to the rheological behavior of the nanocomposite such as solid-like behavior in the low-frequency region and reduction of the Newtonian region. The scanning electron microscopy micrographs revealed that the particle aggregates increase with particle size reduction and increasing nanoparticle content. We also used a nonlinear optimization to obtain the parameters of a multimode Maxwell model for low nanofiller content ABS/SiO2 nanocomposites and found the relaxation times of the polymer chains increased with increasing nanoparticle content. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013