In order to explain the effect of induction hardening on fatigue strength, we tried several fatigue teats by changing. the stress concentration factor αK.We used C=0.41% steel, and chose the three types, as the heat treatments.The following facts were cleared up from the experimental results.(i) At normalized specimens, αK becomes greater than βK, as we have known already.(ii) At ordinary quenching (heating in electric furnace and quenching in water), αK equals approximately, βK.(iii) At induction hardening, the fatigue limits is considerably high, and does not change largely by changing of αK.It was apparent that, within the range of the investigation, the greater αK the larger became the effect of induction hardening on the fatigue limit.Lastly we discussed the reason why the fatigue limits became high by induction hardening.