An exhaust gas post processing system may include an exhaust pipe in which an exhaust gas from an engine passes, a catalyst that may be disposed at the exhaust pipe so as to decrease a harmful material of the exhaust gas, and a water jacket that may be formed around an exhaust pipe that may be disposed at an upstream side of the catalyst of the exhaust pipe, wherein a coolant may be selectively supplied into the water jacket such that the water jacket cools down the exhaust gas passing the exhaust pipe and thus the catalyst does not be over heated.