This study elucidates the behavior of flange plate connection between a steel beam and a welded box column. Four finite element models simulating an exterior connection were prepared and analyzed. On the basis of finite element results, two flange plate connection details which are the reinforcing plate length and plate-to-flange fillet weld geometry were improved. Then, two full-scale specimens with flange plate connections were tested using a standard connection requalification test protocol. The flange plate connections of test specimens achieved the AISC seismic provision requirements for special moment frames.