From the recent fatigue test of the large scale, the fatigue strength of the welded structures used the high strength steels is less than that of the welded structures used the low strength steel. The improvement of the fatigue strength in the high stregth steels is important because the use of the high strength steels in the long span bridges is inevitable. Since the longitudinal welded parts of the plate girder and box girder bridges have no functions of the load transmisson, so the undermatching weld is expected to use. This study is performed to improve the fatigue strength in the logitudinal welded parts by the undermatching weld so to speak the tensile strength of the welding electrods less than that of the base metal. From the test results, the fatigue strength of the longitudinal welded part is improved by the undermatching weld by reason of decreasing of the tensile residual stress, the defect occurrence. the crack occurrence ratio from the defects.