Abstract: A system for cooling internal combustion engines, in which there is provided a main cooling water line, a by-pass cooling water line and a motor-driven valve mechanism for regulating the mixture ratio of the water from the above two cooling water lines. The operation of the motor-driven valve mechanism is controlled by an electric control circuit which receives the signal from a water temperature sensor positioned close to the water inlet to the engine and generates the control signal to be transmitted to the motor. The motor-driven valve has a spool which makes an up-down movement or a rotary movement to regulate the mixture ratio of water.