The purpose of this study is to evaluate the seismic performance of steel box-column connections with electro-slag welding (ESW) stiffeners. Steel box columns have been widely adopted in seismically active areas, such as Japan and Taiwan. It is a common practice to weld diaphragm plates to the inner walls of steel box columns using the ESW process. However, there is still a lack of study evaluating the seismic performance of these steel box-column connections. In the first part of this paper, a literature review examines ESW stiffeners and their impacts on steel box-column connections. In the second part, finite element analysis is conducted on ESW misalignment and eccentricity. The result shows a correlation between connection failures and ESW stiffeners. In addition, ESW fusion diameters can affect the connections more significantly than ESW eccentricities and the thickness of diaphragm plates.