The application of Friedel-Crafts (FC) alkylation reaction to the in situ compatibilization between polystyrene-grafted nano-sized TiO2 (PS-grafted TiO2) and polypropylene (PP) was assessed. The PS-grafted TiO2 was first prepared via dispersion polymerization of styrene (St) using 3-(trimethoxysilyl) propyl methacrylate treated nano-sized TiO2 (MPS-treated TiO2) as cores, and then dispersed within PP by melt blending with different contents of FC catalyst, AlCl3/St, to give PP/PS-grafted TiO2 nanocomposites. The dispersion of PS-grafted TiO2 in PP and interfacial adhesion of PP/PS-grafted TiO2 nanocomposites were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. The results indicated that the dispersion of PS-grafted TiO2 in PP could be improved, with an enhancement of interfacial adhesion between TiO2 and PP, by introducing FC catalyst with adequate concentration, which led to a better thermal stability and resistance to ultraviolet (UV) aging of PP/PS-grafted TiO2 nanocomposites. The effect of FC catalyst on the compatibilization between the PS-grafted TiO2 and PP was discussed based on FC alkylation reaction mechanism. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2755-2763, 2009