An engine cooling system has a coolant control valve unit and includes a cylinder head disposed on a cylinder block. The coolant control valve unit is configured to receive coolant from a coolant outlet side of the cylinder head to control coolant distributed to a heater and a radiator and to control coolant exhausted from the cylinder block. A control unit is configured to determine a heating priority mode according to operation conditions and to substantially open a first coolant passage corresponding to the heater by controlling the coolant control valve unit in the heating priority mode.