In this paper, in order to investigate an accuracy of the equal energy assumption which is often adopted in evaluating their nonlinear maximum displacement response in seismic design, parametric studies on elasto-plastic behavior of steel bridge pier are carried out. First, the comparison between the maximum displacement response of the steel bridge pier evaluated by the equal energy assumption and the one obtained by the elasto-plastic dynamic response analysis shows that there is a quite difference between them. Secondly, an estimation procedure of the maximum displacement response of the steel pier employing an equivalent SDOF system with bilinear restoring characteristics is proposed and its efficiency is confirmed.