Abstract: A screw and nut assembly includes a screw, a nut and a spacer. The nut has a central through hole and an annular fastening portion extending from one end thereof. The annular fastening portion has a plurality of grooves thereon. The spacer has a central hole and an annular concave portion next to the central hole. The screw penetrates the spacer and the nut, and then the nut is tightly screwed to the screw. When the screw and the nut are locked, the nut is moved toward the spacer. The annular fastening portion of the nut is confined by the annular concave portion of the spacer and compressed inward. Through the grooves, the annular fastening portion is compressed inward, so that the inner edge of the annular fastening portion engages with the outer threaded body of the screw tightly to prevent disengagement.