An air-cooled diesel outboard motor comprising an air-cooled diesel engine. The diesel engine includes a vertical crankshaft, an exhausting port and a drive-leg. The drive leg includes a gear-box, a propeller and a water pump in a lower unit, a water cooling exhalation port and a mechanism fastened to the boat. The vertical crankshaft is attached to the drive-leg and an exhalation port of the engine connects to the exhalation port of the drive leg with a water cooled pipe through which the cooling water flows. Propelling power is utilized to push the water into the port.