Octakis(dimethylsiloxypropylglycidylether)silsesquioxane (OG-POSS) was used as a reinforcing agent to prepare polybenzoxazine nanocomposites. Benzoxazine was synthesized using 1,1-bis(3-methyl-4-hydroxyphenyl)cyclohexane, paraformaldehyde and two distinctive aromatic diamines, namely 4,4′-diaminodiphenylmethane (DDM) and 4,4′-diaminodiphenylether (DDE) through Mannich condensation. Polybenzoxazine-POSS hybrid nanocomposites were prepared by ring opening polymerization via thermal curing using benzoxazine precursors and OG-POSS (up to 10 wt %). Thermogravimetric analysis indicate that the nanocomposites display improved thermal stability. Morphological characteristic was studied by scanning electron microscopy. The dispersion of POSS in polybenzoxazine and nanostructure of the composites were confirmed by confocal optical microscopy (COM), transmission electron microscopy (TEM) and atomic force microscopic (AFM) analysis.