9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was immobilized onto the surface of titanium dioxide (TiO₂) nanoparticles in supercritical carbon dioxide. The DOPO immobilized TiO₂ was characterized by UV-visible spectroscopy, magic angle spinning nuclear magnetic resonance and thermogravimetric analysis. Cone calorimeter was used to characterize the flame retardancy properties. By incorporating the DOPO-immobilized TiO₂ nanoparticles (TiO₂-g-DOPO) (5%) into the polyurethane matrix, the heat release rate and total heat release decrease. SEM observation were performed on the combustion residue chars after cone calorimeter test. It is found the char layer is compact for polyurethane with TiO₂-g-DOPO. Apparent activation energies of the polymer composites were estimated via Ozawa method. It is found that the incorporation of TiO₂-g-DOPO improves activation energies. Based on the results, it is speculated that TiO₂-g-DOPO could inhibit the degradation of polyurethane and catalyze the formation of carbonaceous char on the surface. Thus, flame retardancy properties of polyurethane are improved.