A ventilation valve of a hydraulic jack is dedicated to be mounted at an outer wall of an oil storage tank of a hydraulic jack. The hydraulic jack includes a valve seat, a valve ball, and a cap. When the jack is not operated, a valve hole is opened; the vent of the cap is communicated with the oil storage tank of the jack so as to maintain the equilibrium of the pressure of the oil storage tank; thereby the piston of the pump can suck oil successfully so that the oil is absorbed into the oil chamber of the pump to be saturated therein. At the moment that the valve ball ejects upwards rapidly, the oil buffer chamber can buffer high pressure draining oil until the oil return operation of the jack is accomplished.