The low-temperature-quenching (L. T. Q.) is greatly effective on the improvement of the mechanical properties, especially of the fatigue strength, of steel. The effect of the L. T. Q. on the increase of the fatigue strength is more remarkable in specimens with notch than in plain ones. The reason for this is as follows : (a) The strengthening of the ferrite by strain-aging during the repeated loading must be more remarkable owing to the sharp concentrated stress at the root of notch which accelerates the strain-aging. (b) Before the formation of fatigue crack at the root of notch, there appears a region which resists the formation of fatigue cracks. In the low-temperature-quenched specimen, this region is not only expands inward more and more by repeated loading but also strengthened. (c) When a fatigue crack is once formed, the resistance against its propergation is greater in notched specimens than in plain ones.