A thermo-actuator in order to be used for controlling coolant flow through at least one valve seat in a thermostat housing placed in cooling circulation circuits of internal combustion engines and comprising - a cylindrical body that includes a stepped diameter structure embodied in protrusion form at the continuation of mentioned cylindrical body, wherein mentioned stepped diameter structure consists of a side surface, a lower surface and a connection part connecting mentioned side surface and lower surface at a corner, - a valve that is in annular form and fixed onto said cylindrical body in a manner at least partially encircling the body, - a ring-shaped gasket that is positioned between mentioned valve and said stepped diameter as providing sealing, characterized in that mentioned ring-shaped gasket comprises - a lower lip stopped onto mentioned lower surface, - an upper lip extending throughout mentioned side surface, - a contact knuckle extending between mentioned lower lip and the upper lip by encircling the outer surface of mentioned connection part, and wherein said ring-shaped gasket has a stage-like stepped form appropriate to the form of stepped diameter structure, between said upper lip and lower lip thanks to that the diameter of the upper lip is greater than the diameter of the lower lip, and further said valve has a stage-like stepped form appropriate to the form of the ring-shaped gasket and comprises - a holder part that is fixed onto the cylindrical body in a shrink-fit manner in a manner tightening, extends downwards in a manner encircling the body in the longitudinal direction and has a contact surface positioned in a manner at least partially encircling the body - a press part that compresses a seating surface, provided on the lower lip, by means of exerting pressure towards said lower surface, - a grabbing part that compresses the contact knuckle by means of exerting pressure towards mentioned connection part, - a stop part that compresses the upper lip by means of exerting pressure towards said side surface and wherein the valve crushes the ring-shaped gasket between the valve and said stepped diameter in order to improve sealing between them. - a cylindrical body that includes a stepped diameter structure embodied in protrusion form at the continuation of mentioned cylindrical body, wherein mentioned stepped diameter structure consists of a side surface, a lower surface and a connection part connecting mentioned side surface and lower surface at a corner, - a valve that is in annular form and fixed onto said cylindrical body in a manner at least partially encircling the body, - a ring-shaped gasket that is positioned between mentioned valve and said stepped diameter as providing sealing, characterized in that - a lower lip stopped onto mentioned lower surface, - an upper lip extending throughout mentioned side surface, - a contact knuckle extending between mentioned lower lip and the upper lip by encircling the outer surface of mentioned connection part, - a holder part that is fixed onto the cylindrical body in a shrink-fit manner in a manner tightening, extends downwards in a manner encircling the body in the longitudinal direction and has a contact surface positioned in a manner at least partially encircling the body - a press part that compresses a seating surface, provided on the lower lip, by means of exerting pressure towards said lower surface, - a grabbing part that compresses the contact knuckle by means of exerting pressure towards mentioned connection part, - a stop part that compresses the upper lip by means of exerting pressure towards said side surface