This investigation was performed to clarify the diagram of endurance limit of notched specimens under repeated torsion. The materials used were of two kinds of carbon steels, and specimens had V-grooves of two or three kinds of sharpness.Results obtained are as follows:(1) In this experiments the authors defined the fatigue limit as a maximum reversed stress at which cracks do not yield at the root of notch. Such a determination of fatigue limit is more appropriate theoretically than such case that a maximum reversed stress, at which the specimen does not break down, is defined as the fatigue limit. By such determination, the diagrams of endurance limit under reversed torsion were procured for a few notched specimens.(2) The effect of mean stress in the diagram of endurance limit was much considerable in this experiments than in early experiments basing upon whether the specimen break down or not. Further it was cleared from this results that the diagram of endurance limit of notched specimens under reversed torsion becomes a line drawn from the completely reversed fatigue limit, on the ordinate, to a point on the abscissa, obtained from dividing the torsional strength of unnotched specimen into the stress concentration factor α.(3) Above tendency was understood from consideration of the distribution of mean stress in the specimen during the fatigue tests.