Thermostat assembly with tandem valve for a coolant circulation system characterized by, • at least one body, • at least one by-pass output which is in fact a hose connection and which is used for connection to a by-pass circuit, • at least one inlet which is in fact a hole on the bottom side of body and which is used for transferring the coolant from an engine block, • at least two valve seats which form together a cylindrical geometry protruding from the body and which allow a double-sided flow entrance into flow control construction, • at least one hollowed tandem valve, the outside of it has a hyperboloid shape and the bottom and the top contact surface of it has a frusto-conical shape and which is installed inside of the body, • at least two elastomer sealing rings which are installed to channels on the joint faces between the hollowed tandem valve and valve seats leading to the coolant outlets inside of the body to provide sealing, • at least one support face, which is extending over the hollowed tandem valve and has a hollow shape, • at least one by-pass valve which is slider type and is attached to connector extension to control the coolant flow from by-pass output, • at least one connector extension which is connected to hollowed tandem valve for driving by-pass valve, • at least one radiator output which has suitable connectors for connecting the cooling circuit and for welding the mounting part on the body, • at least one thermal actuator which is attached on support face, generates the required force to move the hollowed tandem valve and slider type by-pass valve down related with the temperature, • at least one spring which apply force to the thermal actuator on opposite direction proportionally with the tension caused by the force generated by the thermal actuator, • at least one spring retainer which provides housing for the spring and the thermal actuator by means of the cylindrical hole and which is installed between the cover and the body, • at least one piston support extension with guide ribs which is created by protruding close end of the body and which provides a coaxial movement for the hollowed tandem valve by means of the engaging structure with the pin of thermal actuator. • at least one body, • at least one by-pass output which is in fact a hose connection and which is used for connection to a by-pass circuit, • at least one inlet which is in fact a hole on the bottom side of body and which is used for transferring the coolant from an engine block, • at least two valve seats which form together a cylindrical geometry protruding from the body and which allow a double-sided flow entrance into flow control construction, • at least one hollowed tandem valve, the outside of it has a hyperboloid shape and the bottom and the top contact surface of it has a frusto-conical shape and which is installed inside of the body, • at least two elastomer sealing rings which are installed to channels on the joint faces between the hollowed tandem valve and valve seats leading to the coolant outlets inside of the body to provide sealing, • at least one support face, which is extending over the hollowed tandem valve and has a hollow shape, • at least one by-pass valve which is slider type and is attached to connector extension to control the coolant flow from by-pass output, • at least one connector extension which is connected to hollowed tandem valve for driving by-pass valve, • at least one radiator output which has suitable connectors for connecting the cooling circuit and for welding the mounting part on the body, • at least one thermal actuator which is attached on support face, generates the required force to move the hollowed tandem valve and slider type by-pass valve down related with the temperature, • at least one spring which apply force to the thermal actuator on opposite direction proportionally with the tension caused by the force generated by the thermal actuator, • at least one spring retainer which provides housing for the spring and the thermal actuator by means of the cylindrical hole and which is installed between the cover and the body, • at least one piston support extension with guide ribs which is created by protruding close end of the body and which provides a coaxial movement for the hollowed tandem valve by means of the engaging structure with the pin of thermal actuator.