A connection of hollow structural section (HSS) has complicated details according to the connection type, and shows a very complex local deformation at the yield stress. Such various type of connections, Korea Building Code (KBC 2009) has suggested simple design equations because it was difficult to consider the complex local deformation. KBC 2009 has respectively proposed the design equations of the connections under axial force or moment, but it has not proposed the design equations of the connections under axial force and moment, simultaneously. In this paper, numerical analysis of the connection under combined load was carried out using the model based on the finite element analysis to ensure the reliability analysis.