In this work, the reaction to fire of polymer nanocomposites (thermoplastic polyurethane, polylactide and polyamide-6) containing different nanofillers (organoclay, polyhedral silsesquioxanes or POSS and carbon nanotube) is investigated. When high level of nanodispersion is achieved (shown by transmission electron microscopy (TEM)), they exhibit good flame retardancy in specific scenarii (high heat flux), but fail to flammability tests (LOI, UL-94). The mechanism of protection is the formation of mineral layer associated to char promotion but the protective coating is not efficient enough to provide the highest standard of protection. It is shown that this technology gives the best results combined with conventional flame retardants and leads to synergistic effects. The aspects of nanodispersion of the filler with the flame retardant are also fully commented in the paper using TEM and electron microprobe.