A cooling system for a power transmission unit that can supply oil properly to a differential mechanism and motors. The cooling system comprises: a first branch passage to supply the oil to the differential mechanism; a second branch passage to supply the oil to the first motor; and a third branch passage to supply the oil to the second motor. The second branch passage includes a first oil feeding section that supplies the oil to a coil of the first motor, and a second oil feeding section that supplies the oil f to a core of the first motor. The third branch passage includes a third oil feeding section that supplies the oil to a coil of the second motor, and a fourth oil feeding section that supplies the oil to a core of the second motor.