A cooling system of an internal combustion engine is provided that can improve flexibility of arrangement of peripheral machinery included in the engine. A cooling system of an internal combustion engine includes a crankcase; a cylinder block disposed on an upper portion of the crankcase and having a plurality of cylinder bores; a cylinder head disposed on an upper portion of the cylinder block; and an oil passage formed in a rear surface portion of the cylinder block so as to extend along a cylinder-arrangement direction. The oil temperature sensor is attached to the oil passage from the axial direction of the oil passage.