We obtained the following results from fatigue test on the welded joints of HT 80 with the reinforcements removed using the modified Schenck's test pieces. (1) The endurance limits of the welded joints were slightly lower than that of the base metal. (2) Most welded joints failed at the bonded zone or the softened zone in H. A. Z. In the case of the specimens failed at the bonded zone, the fatigue cracks started in the coarse grain zone. (3) The welded joints failed almost at the softened zone when the applied stress was comparatively high and failed slightly more at the bonded zone than the softened zone when the applied stress was near the endurance limit. (4) Considering the result (3), we had investigated the endurance limit (σ) from two points hardness ( H ) and grain diameter ( D ) and proposed the following formula, σ= a ₁ H 1/√ D + b ₁1/√ D + a ₂ H + b ₂, where a ₁, b ₁, a ₂, b ₂ are the constant values without concerning the material, and we proposed the following values : a ₁=0.0017, b ₁=0.54, a ₂=0.11, b ₂=3.5. (5) From one more experiment on the grain size dependence of the fatigue strength, we obtained the fact that the effect of the grain size on the fatigue strength decreases with increase of the applied stress. (6) The equation above mentioned shows the endurance limit, but when the applied stress is comparatively higher than the endurance limit, the values ' a ₁' and ' b ₁' must be smaller than 0.0017 and 0.54 judging from the result (5). In this case we can see that the fatigue strength of bonded zone is higher than that of the softened zone.