High modulus and thermally stable clay/polyimide (pyromellitic dianhydride-4,4′-oxydianiline, PMDA-ODA) nanocomposites were synthesized from reactive organoclay and poly(amic acid). The reactive organoclay was formed by using p-phenylenediamine as a swelling agent for silicate layers of montmorillonite. The swelling process was first carried out through ion exchange between the onium of one amine end group of p-phenylenediamine and the sodium ion in montmorillonite. The other amine end group of p-phenylenediamine reacted with the dianhydride end group of poly(amic acid), producing an irreversible swelling. This irreversible swelling resulted in a nanostructured material containing intercalated nanometer silicate layers dispersed in PMDA-ODA as confirmed by both X-ray and transmission electron microscopy. The glass transition temperature and the thermal decomposition temperatures of this type of clay/PMDA-ODA are higher than that of pure PMDA-ODA. A 2.5-fold increase in the modulus of clay/PMDA-ODA film was obtained as compared to that of pure PMDA-ODA film. Both the maximum stress and the elongation-for-break of these clay/PMDA-ODA nanocomposites increased with the amount of organoclay. The maximum reduction in out-of-plane coefficient of thermal expansion of this type clay/PMDA-ODA was 50% as compared to that of pure PMDA-ODA.