The organic-inorganic nanocomposites involving OPBI (poly[2,2'-(p-oxidiphenylene)-5,5'-bibenzimidazole]) and polyhedral oligomeric silsesquioxane (FOSS) were prepared via in situ polymerization of 4,4'-dicarboxydiphenyl ether (DCDPE) and 3,3'-diaminobenzidine (DABz) in the presence of the POSS where the organic group on silsesquioxane cage is phenyl. NMR, XPS, and XRD characterization of POSS-g-OPBI, purified from the POSS-g-OPBI/OPBI hybrid, revealed that the OPBI chains were successfully attached to the phenyl group of the FOSS through Friedel Crafts (F C) reaction. The homogeneous dispersion of FOSS cages in the polymer matrix was evidenced by SEM. The DMA results showed that the moduli of the POSS-g-OPBI/OPBI nanocomposites were significantly higher than OPBI matrix, indicating the nano-reinforcement effect of POSS cages. Thermogravimetric analysis indicated that the thermal stability of the polymer matrix was not sacrificed but improved by introducing a small amount of POSS since POSS showed lower thermal stability than OPBI. More importantly, the mechanical properties, including tensile and yield strength, Young's modulus, and toughness, were obviously simultaneously increased by introducing FOSS into the nanocomposite, which is quite differently from the traditional nanocomposites, where the ductility and toughness of polymer were usually reduced substantially upon the incorporation of inorganic reinforced agent.