A device for the fixing of a prismatic mechanical test piece, which consists of two mechanisms that secure the test piece (7) on both sides, where each mechanism consists of two independent blocks (1 and 2), the second block (2) being displaceable in relation to the first (1), at least two rods (3) and a cylinder (4) for assisting the attachment between both blocks (1 and 2), a screw (5) for assuring the attachment between both blocks (1 and 2), and an adapter plate (6) which is adjusted to the second block (2) by means of a pin or bolt (8). Moreover, both blocks (1 and 2) have on the surface facing the test piece (7), an inclined plane (9) which has a grinding at each end in the form of a dovetail (10), such that the test piece (7) remains secured between the two grindings (10).