A novel technique of surface modification was used to treat nanomodified aluminum trihydrate (nano-CG-ATH). The results of the surface modification were characterized with transmission electron microscopy and Fourier transform infrared spectra. The effects of the surface modification on the properties of high-impact polystyrene (HIPS) composites were studied with limiting oxygen index (LOI) and mechanical tests. The dispersion of nano-CG-ATH in the HIPS matrix and the interfacial adhesion between them were observed with transmission electron microscopy and scanning electron microscopy. The experimental results demonstrate that the surface of nano-CG-ATH was successfully grafted by an organic substance, and the dispersion of treated nano-CG-ATH in ethanol was better than that of untreated nano-CG-ATH. At high loadings, the mechanical properties and LOI values of the HIPS composites with treated nano-CG-ATH were higher than those of the HIPS composites with untreated nano-CG-ATH. The dispersion of treated nano-CG-ATH in the HIPS matrix was better than that of untreated nano-CG-ATH in the HIPS matrix. Also, the interfacial adhesion between the HIPS matrix and treated nano-CG-ATH was better than that between the HIPS matrix and untreated nano-CG-ATH. (c) 2008 Wiley Periodicals, Inc. I Appl Polym Sci 112: 359-365, 2009