Abstract: A cooling liquid temperature control system for an internal combustion engine cooled by a cooling liquid which in turn is cooled by a heat exchanger, wherein a primary bypass opened and closed by a low operating temperature thermostat valve and a secondary bypass opened and closed by a control valve are branched from a cooling liquid outlet passage connected between an engine jacket and the heat exchanger. The control valve can be opened depending on the degree of the opening of the throttle valve. The secondary bypass is branched from the cooling liquid outlet passage. The two passages are respectively opened and closed by a high operating temperature thermostat valve. The secondary bypass is communicated with the primary bypass and the other passage communicated with the heat exchanger.