Abstract: Disclosed is a piston-stroke adjusting mechanism of a hydraulic tool comprising an oil tank, a ram for causing the forced feed of an oil, and a piston permitted to slide by the pressure of a working fluid. The piston is formed with a through bore into which an adjust pin is slidably inserted in a manner that an end thereof is directed toward the bottom of a cylinder. The piston is rotatably mounted with an adapter at its head section. The adapter has a small-diameter portion at one side thereof which is located in the through bore to be connected to the adjust pin.