This paper describes a simple semi-active control algorithm in which the Magneto-Rheological fluid (MR) damper's hysteresis shape is controlled. The proposed method aims at reducing a displacement without increasing an acceleration response. The first part of this paper presents the basic properties of the proposed method obtained by analytical and numerical means using a S-DOF system. Next, it describes a series of seismic response analyses carried out in order to verify the applicability of the proposed method to base-isolated structures, It was confirmed that the proposed method is effective in improving the performance of the base-isolated structure.