Polypropylene (PP)/octavinyl polyhedral oligomeric silsesquioxane (OvPOSS) composites were prepared by physical blending or reactive blending methods. The comparison of the PP/OvPOSS composites prepared by these two methods was investigated by mechanical tests, thermogravimetric analysis, and cone calorimeter. The graft ratio of OvPOSS to PP chain increased with increasing OvPOSS and dicumyl peroxide content for the reactive blending composites. The reactive blending composites had better mechanical properties, thermal stability than physical blending composites. The peak of the heat release rate and mass loss rate of PP/OvPOSS had also decreased, indicating better flame retardancy of PP/OvPOSS composites. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3745-3751, 2008