Abstract: A nut includes a body with a first through hole and a disk formed on a bottom face of the body and having multiple first securing elements formed on a bottom face of the disk. A washer includes an annular disk having a second through hole, multiple second securing elements to securely engage with the first securing elements, multiple abutting elements formed on a bottom face of the annular disk for securing engagement with a workpiece surface. Therefore, when the nut and the washer are combined, engagement between the first securing elements and the second securing elements facilitates the tightening process to a workpiece. Each of the first securing elements and of the second securing elements has an inclination direction opposite toward a direction of the abutting elements.