Disclosed is a radiator cooling structure for cooling a radiator of a water-cooled power unit mounted on a small vehicle and disposed on a laterally outer side of the small vehicle, which radiator cooling structure improves the efficiency of cooling an internal combustion engine by using a head wind. The radiator is provided with a radiator cover having a louver for guiding cooling air to the radiator. The louver is formed such that a rear part thereof is more protruded laterally outward than a front part thereof. The louver of the radiator cover has parallel slats tilting forward, and a lower body cover is provided with a cooling air intake opening capable of guiding a head wind toward the louver positioned obliquely above and rearward of the cooling air intake opening.