This invention relates to thermostat assembly with tandem valve structure for cooling circulation systems of internal combustion engines which comprises a solid tandem valve (4) or hollowed tandem valve (5) to control the flow of the coolant fluid by opening or closing the 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 of this a mechanism with high flow rates and with low pressure drops in limited geometries becomes obtainable.