The slenderness ratio parameter of steel members such as arch ribs is larger than that of usual steel bridge piers. However, previous studies on the seismic performance of steel members with the large slenderness ratio parameter are not sufficient compared with usual steel bridge piers. The purpose of this study is to obtain more results on the load-carrying capacity and the ductility for steel members with the relatively large slenderness ratio parameter based on the elasto-plastic finite element analysis. The validity of the analytical method was confirmed by comparing the analytical results with the previous experimental results. Based on the valid analytical method, the parametric analysis about the width-thickness ratio parameters was conducted and the effect of the slenderness ratio parameter was investigated. Analysis results indicate that the load-carrying capacity and the ductility for steel members decrease as the slenderness ratio parameter increases or the width-thickness ratio parameters increase.