In this paper, the effects of the clamping force and the pre-clamping force on the fatigue strength for the M 4 rolled-bolts were described theoretically and experimentally. The important items obtained in this paper were listed as follows. (1) The experimental value of the fatigue limit for the optimum clamping force in this paper increased about 43% as compared with the fatigue limit for the conventional clamping force, at the mean stress σm=33 kgf/mm2. (2) In the case of the test bolts it can be made a rough estimate of the optimum clamping force by the theoretical value of n''= 1/2. (3) The experimental value of the fatigue limit for the optimum clamping force after the pre-clamping increased about 10% as compared with the fatigue limit for the optimum clamping force. (4) The reasons for the improvement of the fatigue limits with the pre-setting to the rolled-bolts were not only the residual stress arising at the root bottoms of thread and the work hardening but also the effects of the optimum clamping force after the pre-clamping.