Cooling circuit of engine

A cooling circuit of an engine comprising:

• a water pump provided at a coolant circulation line;
• an engine provided with a water jacket forming a pathway through which a coolant supplied from the water pump passes, and having a coolant inflow line and a coolant exhaust line connected to the pathway of the water jacket;
• a radiator connected to the coolant exhaust line of the engine;
• a thermostat selectively connecting the radiator with the water pump so as to control a coolant flow therebetween;
• a first cooling line connecting the coolant exhaust line to a heater;
• a first return line connecting the heater to the water pump through the thermostat;
• a second cooling line connecting the coolant exhaust line to a throttle body;
• a third cooling line connecting the throttle body to an oil cooler;
• and
• a second return line connecting the oil cooler to the first return line so as to guide a coolant exhausted from the oil cooler to the first return line.