A cooling circuit of an engine may include 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, a radiator connected to the coolant exhaust line, a thermostat selectively connecting the radiator with the water pump to control a coolant flow, 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 to guide a coolant exhausted from the oil cooler to the first return line.