A series of functional polyhedral oligomer silsesquioxnae, (POSS)/polyimide (PI) nanocomposites were prepared using a two-step approach, first, the octa(aminophenyl)silsesquioxane (OAPS)/NMP solution was mixed with polyamic acid (PAA) solution prepared by reacting 4,4'-diaminodiphenylmethane and 3,3',4,4'-benzophenonetetracarboxylic dianhydride in NMP, and second, the polycondensation solution was treated by thermal imidization. The well-defined 'hard particles' (POSS) and the strong covalent bonds between the PI and the 'hard particles' lead to a significant improvement in the thermal mechanical properties of the resulting nanocomposites. The glass transition temperature dramatically increases while the coefficient of thermal expansion (CTE) decreases, owing to the significant increase of the cross-linking density in the PI-POSS nanocomposites. The thermal stability and mechanical property of the nanocomposites were also improved. (C) 2003 Published by Elsevier Science Ltd.