An epoxyfunctional cyclotriphosphazene with silicon moieties and bulky aromatic substituents was employed for modification of polyurethane high-solid coatings. The structure of compounds was characterized by NMR and FTIR spectroscopy. The effect of cyclotriphosphazene on the thermal stability and flame retardancy of hybrid organic-inorganic poly(urethane-cyclophosphazene)s was studied. Some surface properties (gloss, water contact angle, chemical resistance) and performance properties (hardness, elasticity, impact resistance, adhesion and cupping) of the modified coatings were also investigated. The introduction of a cyclotriphosphazene resulted in an increase in glass transition temperature and thermal stability. With a 1 % addition of the modifier, a significant decrease in heat release rate (HRR) by 10 % is observed. A lower rate of heat release may be due to the formation of a protective layer over the bulk of the sample and prevention of further combustion, making the thermal degradation process slower. Furthermore, coatings show excellent elasticity (Φ 2 mm), impact resistance (50 cm), adhesion to steel (0), and cupping (>9.5 mm); however, their relative hardness is rather low (0.06–0.10). The gloss of poly(urethane-cyclophosphazene)s was relatively high (approx. 100 GU at 60°), but it decreased considerably for the sample with 20 % by weight of the modifier. The introduction of cyclotriphosphazene resulted in a decrease in the hydrophobicity of the coatings.