We synthesized double-decker silsesquioxane (DDSQ)-functionalized benzoxazine (BZ) monomers from a DDSQ prepared with nadic anhydride (ND) through hydrosilylation (to form DDSQ-ND) and then its reactions with p-aminophenol and o-aminophenol to form DDSQND-p-OH and DDSQND-o-OH, respectively. Four different DDSQ-functionalized BZ monomers were prepared through reacting DDSQ-ND-p-OH and DDSQND-o-OH with CH2O and either aniline or allylamine: pBDDSQ-AN, pBDDSQ-AL, oBDDSQ-AN, and oBDDSQAL. All of these polybenzoxazine (PBZ)/DDSQ hybrids exhibited high thermal stability and high char yields after thermal curing, based on thermogravimetric analysis (TGA), because the DDSQ inorganic nanoparticles were dispersed homogeneously in the PBZ matrices, as evidenced using electron microscopy. For example, the thermal decomposition temperature (T-d) and char yield of pBDDSQ-AN after thermal curing at 270 degrees C were 521 degrees C and 75 wt %, respectively. More interestingly, the char yield of the ortho-substituted oBDDSQ-AN increased from 72.7 wt % prior to thermal treatment to 76.0 wt % after thermal treatment at 450 degrees C, indicating that the polybenzoxazole may have formed after such thermal treatment to increase the char yield, which was higher than that of the para-substituted pBDDSQ-AN.