A fuel cooling system for a motorvehicle has a fuel tank for supplying fuel to a motorvehicle engine. The system further includes a refrigerant evaporator and a compressor of a refrigeration system for air conditioning, and a heat exchanger provided between a fuel pipe and an evaporated refrigerant pipe. The heat exchanger is made up of coaxial inner and outer tubes and, for example, helical heat transfer fins contained in an annular space between the inner and outer tubes. With this construction, the fuel flowing through a fuel return pipe extending between the engine and the fuel tank is caused to flow through the annular space between the inner and outer tubes. On the other hand, evaporated low temperature refrigerant is caused to flow through the inside of the inner tube of the heat exchanger. The inner tube has secured therein, heat exchange fins, for example, of the type extending longitudinally thereof and having wavy transverse cross section. The fuel and the refrigerant exchange heat through the inner tube, whereby the fuel is cooled effectively.