The objective of this study is to assess the local elasto-plastic behavior of a beam-to-column connection between a concrete-filled square steel tube column and a steel beam, used in a perimeter moment resisting frame system. The connection is equipped with a side stiffener and a divided external diaphragm. This paper describes experimental results and an analytical model for predicting the load-displacement relation associated with this beam-to-column connection. Parameters investigated in the tensile tests include the side stiffener configuration and the inplane beam eccentricity. The analytical model is developed by superposing the strength of the side stiffener and external diaphragm on the load-displacement relations for a tube flange. The tube flange is modeled as a grid beam with a tetra-linear load-displacement relation. The analytical method is found to agree approximately with the experimental results up to large deformations.