A locking device for mechanical pieces, particularly for pieces subjected to mechanical processing or similar, comprising at least one hollow body that forms a sliding seat, a movable stem that is accommodated slidingly partially within the sliding seat of the hollow body and radial expansion elements which are interposed between the hollow body and the movable stem, the radial expansion elements being adapted to mutually lock the hollow body and the movable stem, the movable stem protruding partially from the hollow body with the hollow body and the movable stem jointly associable, at their free ends, by way of elements for connection, respectively, to a mechanical piece to be processed or a fixed part of a machine with which the mechanical piece is to be processed.