This experimental research was made to clarify the effects of penetration depth and bead form of fillet welding on the fatigue strength of welded joint.Front fillet welded joint (cross type) was adopted to the specimens in this experiment, and two kinds of electrode were used to clarify the effect of penetration depth and bead form.Steel plates were of JIS SM 41 W carbon steel of 12mm thickness, and surface of specimens were mill scale on.The welding electrodes were high iron oxide type A, and B which had slight differences in characteristics and chemical composition, and the diamater of core wire was 6.4mm, both types can produce deep penetration weld.The welded specimens were produced by one pass fillet weld, at room temperature. The specimens were tested in the as-welded condition (reinforcement on, not stress-relieved). Fatigue tests were conducted by the Losenhausen fatigue testing machine (UHS type) under the pulsating stresses of 0-+σmax, and +8.5kg/mm2-+σmax, at room temperature (appoximately 24°C).The main experimental results of this research are as follows.(1) The series of specimens connected by the electrode A which has medium penetration depth, that is shallower as compared with B, has higher fatigue strength than in the case of specimens connected by the electrode B.(2) The specimens connected by electrode A fractured at the heat affected zone of mother metal, and those connected by electrode B fractured at the weld metal zone of welded joint.(3) It seems that the fracture occurred in the mother plate was caused mainly by the rolling structure.