In this study, the authors investigated the fatigue crack initiation life N _c at the toe of butt welded joint and at the toe of cruciform butt welded joint subjected to pulsating tensile load with the cycling rate of 10 cpm both in sea water and in air. The results of the tests were compared with those estimated according to the concept proposed in the previous paper, in which a method of estimation of the fatigue crack initiation life N_c of a notched mild steel plate both in sea water and in air was shown on the basis of the stress-strain function D . As a result of this study, the following conclusions were obtained : (1) The corrosion effect factor K_c at the toe of manual arc welded joints in sea water is nealy equal to that in, the notched mild steel base plates in sea water. (2) The fatigue crack initiation life of the toe of manual arc welded joint of a mild steel under the corrosive environment of sea water in ambient temperatures can be estimated by using the results of completely reversed strain controlled fatigue tests of smooth round bar specimen of base metal in air.