Some torsional fatigue tests on the models of built-up flange coupling were carried out to obtain the strength of the shaft joint subjected to repeated torsion. These models were consisted of 70φ forged mild steel shafts and forged steel flanges. The flange was fitted forcedly with a nut on the tapered part of the shaft having a key. The following results were obtained through the tests which were carried out under the different kinds of conditions of fitting and loading. (1) When the flange was fitted hard on the tapered surface of the shaft, a crack originated from the end corner of the key way and caused the failure of the shaft. The torsional fatigue limit strength was 10kg/mm², and the fatigue, notch factor based upon 60φ straight shaft was 1.45. (2) In the looser fitting condition, the fitted surface suffered fretting corrosion, and the shaft was failed by the cracks originated in the corrosion. The fatigue strength at 2×10⁷ cycle was presumed about 8kg/mm², (3) In the case of looser fitting and the initial torque being added, the cracks occured along the bottom corner of key way, at which stress was rised by pulsatory thrust of key and the damage of surface at just outer side of the key way. The fatigue limit strength may be lower than 7kg/mm²when mean stress is 4kg/mm².