An engine cooling system, comprising: a selective catalytic reduction device installed in an exhaust pipe through which an exhaust gas from an engine is exhausted and configured to reduce a nitrogen oxide in the exhaust gas; a reducing agent injection module connected to the exhaust pipe configured to inject a reducing agent into the exhaust pipe upstream of the selective catalytic reduction device , the reducing agent injection module having a module cooling channel therein; a coolant circulation line including a coolant supply line for supplying a coolant to the module cooling channel and a coolant recovery line for recovering the coolant heated in the module cooling channel ; a coolant recovery portion installed higher than the reducing agent injection module along a gravitational direction configured to store the coolant discharged through the coolant recovery line, the coolant recovery portion including a vapor discharge outlet for discharging the coolant in vapor state; an engine coolant circulation line configured to circulate the coolant for cooling an engine and including an engine coolant supply line and an engine coolant recovery line ; a radiator configured to cool the coolant flowing through the engine coolant recovery line and to supply the coolant to the engine coolant supply line ; and a vapor storage portion positioned over the radiator along the gravitational direction configured to store the coolant in vapor state discharged from the radiator , wherein the coolant supply line and the coolant recovery line are connected to the engine coolant circulation line such that at least a portion of the coolant circulating through the engine coolant circulation line is supplied to the module cooling channel , and wherein the vapor storage portion is connected to the vapor discharge outlet through a vapor discharge line to store the coolant in vapor state discharged from the engine coolant circulation line and the module cooling channel respectively. a selective catalytic reduction device installed in an exhaust pipe through which an exhaust gas from an engine is exhausted and configured to reduce a nitrogen oxide in the exhaust gas; a reducing agent injection module connected to the exhaust pipe configured to inject a reducing agent into the exhaust pipe upstream of the selective catalytic reduction device , the reducing agent injection module having a module cooling channel therein; a coolant circulation line including a coolant supply line for supplying a coolant to the module cooling channel and a coolant recovery line for recovering the coolant heated in the module cooling channel ; a coolant recovery portion installed higher than the reducing agent injection module along a gravitational direction configured to store the coolant discharged through the coolant recovery line, the coolant recovery portion including a vapor discharge outlet for discharging the coolant in vapor state; an engine coolant circulation line configured to circulate the coolant for cooling an engine and including an engine coolant supply line and an engine coolant recovery line ; a radiator configured to cool the coolant flowing through the engine coolant recovery line and to supply the coolant to the engine coolant supply line ; and a vapor storage portion positioned over the radiator along the gravitational direction configured to store the coolant in vapor state discharged from the radiator , wherein the coolant supply line and the coolant recovery line are connected to the engine coolant circulation line such that at least a portion of the coolant circulating through the engine coolant circulation line is supplied to the module cooling channel , and wherein the vapor storage portion is connected to the vapor discharge outlet through a vapor discharge line to store the coolant in vapor state discharged from the engine coolant circulation line and the module cooling channel respectively. <EOS>