Abstract: A screw-bolt having crest-cut-off threads with their heights progressively decreased to not more than 3/4 times the basic height in the screwing up direction at a gradient ranging from 60/100 to 3/100 over that of the axial length of the threaded part of the bolt which is located between the unthreaded part and the complete thread upon setting up engagement with a nut therefor to allow a fraction of 0.1 to 0.2 unengaged with the nut, whereby the fatigue strength is increased in terms of the endurance limit from 6 kg/mm.sup.2 for the prior art bolt and nut to more than 11 kg/mm.sup.2 independently of whether SCM 14 or SNCM 5 steel is used in making the bolt and nut.