An engine cooling system includes: an engine system including an engine having a plurality of combustion chambers that generate a driving force by combustion of fuel; a power electronics (PE) device that assists engine torque of the engine system in accordance with a driving state of the vehicle; a main radiator connected to the engine system by an engine coolant line and discharging heat generated in the engine system by engine coolant flowing in the engine coolant line, a sub-radiator connected to the PE device through a PE coolant line and discharging heat generated in the PE device by PE coolant flowing in the PE coolant line; a 2-way valve connecting the engine coolant line and the PE coolant line to each other; and a controller for controlling opening and closing of the 2-way valve according to a temperature of the engine coolant and a vehicle driving condition.