In steel bridges, the use of single-sided weld is sometimes unavoidable. Meanwhile, the construction of scallops has been performed as one of emergency measures to overcome crack problems. However, cutting scallops within the single-sided weld may lead to another crack problem due to the existing root in the single-sided weld between the connected parts. In this study, with a case of Minatogawa Bridge, the fatigue durability of the single-sided welded joint of a corner plate after a scallop is constructed was investigated. A series of cyclic bending test of test specimens was conducted to examine the fatigue improvement as well as the effectiveness of HiFIT (High Frequency Impact Treatment) and ICR (Impact Crack-closure Retrofit) treatment as peening techniques. The fatigue test result showed that the treatments increased the fatigue life of the single-sided welded joint.