A power unit for use as a pressure-fluid-operated motor and/or a pressure fluid pump, the power unit comprising a cylinder space, a piston moving in the cylinder space and channels for pressure fluid. The cylinder is annular and it comprises pistons which are of the same shape as the cross section of the cylinder space, and the pistons are arranged to rotate around the axis of the cylinder space. The power unit further comprises a transmission shaft and locking members for locking the piston so that it cannot rotate with respect to the cylinder space.