Trialkyl(pentafluorocyclotriphosphazenyl)ammonium bis(trifluoromethylsulfonyl)imide were synthesized. They were nonflammable room-temperature ionic liquid which showed low ionic conductivity (0.29-2.32 mS cm(-1)) and viscosity (33.3-56.2 mPa s). They displayed extremely low viscosity in spite of the high molecular weight.