A method for mounting a jaw die plate of a jaw crusher detachably against a front end wall of the crusher by means of a fastening device operated from outside a side wall of the crusher. The die plate is secured in its place by means of friction between the fastening device and a counter surface of the front end wall and friction between the fastening device and a counter surface of the die plate. The fastening device of the jaw crusher includes a wedge piece having a first pressing surface to be fitted against the counter surface of the die plate secured against the front end of the crusher, and, in addition to that, a second pressing surface to be fitted against the counter surface of the front end wall. The wedge piece is movable towards both counter surfaces and away from the same by means of the tightening means.