Abstract: This invention relates to a thermostat assembly with tandem valve for cooling circulation systems of internal combustion engines which includes a solid tandem valve or a hollowed tandem valve to control a flow of a coolant fluid by opening or closing an output connection by allowing double side flow entrance of flow into the valve construction to achieve low pressure drops and due to this double side entrance of flow it is possible to activate the mechanism with lower forces. As a result, a mechanism with high flow rates and with low pressure drops in limited geometries becomes obtainable.