A radiator additive which can significantly improve automobile fuel efficiency by adding only slightly to a coolant of a radiator is newly provided. The radiator additive according to the present invention is a radiator additive including a colloidal solution that includes platinum nanoparticles and/or gold nanoparticles having an average particle diameter of 1 to 10 nm, and can improve fuel efficiency of an internal combustion engine by adding to the coolant so as to constitute at least 1% of the volume of the coolant added into the radiator for cooling the internal combustion engine.