Polyurethane/polyhedral oligomeric sisesquioxane (PU/POSS) nanocomposites were synthesized via polymerization utilizing the compatibility between POSS nanoparticles and 4,4′-diphenyl methylene diisocyanate. Scanning electron microscope images and Fourier transform infrared spectra revealed that POSS nanoparticles were dispersed in PU matrix. Thermal gravimetric analysis was employed to investigate the thermal decomposition behavior of PU/POSS nanocomposites at elevated temperatures. Then fire performance was evaluated by limiting oxygen index, underwriters laboratories 94 testing and char residue morphology. These results showed that the addition of POSS promoted the formation of char residues which were covered on the surface of polymer composites, leading to the improvement of thermal stability and flame retardancy.