A thermostat having improved reactivity may include a thermostat case having side connected to first and second passages, and a third passage formed between the first and second passages in a side direction, a valve body where first and second valves are formed to open/close the first and second passages, and a mounting space is formed in the valve body, an elastic member elastically pushing the valve body in a direction of the first valve such that the first valve closes the first passage, and a drive portion moving the valve body in response to contraction or expansion of a wax disposed in the mounting space. A flowing turn wall may be formed to correspond to the third passage such that a liquid flowing in through the second passage moves in a direction opposite to the third passage.