Recently, in the metropolitan expressway, fatigue cracks on steel bridge bents have been reported. The purpose of this study is to clarify the fatigue strength and improve the fatigue strength of beam-circular column connections of steel bridge bents. Fatigue tests and FEM analyses were carried out on large scale specimens by reproducing actual beam-circular column connection. It was found that fatigue performance of that connection has low fatigue strength due to high stress concentration and existence of inherent weld defects at corner of that connection. As a retrofit result of rib-installation, although there is existence of inherent weld defects, rib-installation could improve fatigue strength of that connection.