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.