Many researchers have attempted to apply semi-active control systems in the civil engineering structures. Semi-active control systems offer the reliability of passive control systems, yet provide the adaptability of active control systems. Recently, magneto-rheological (MR) dampers have been developed. MR fluid dampers can be controlled with small power supplies and the dynamic range of the damping force is quite large. This MR damper is one of semi-active dampers as a new class of smart dampers. This paper discusses the application of MR damper for seismic response control of adjacent buildings subjected to earthquake. Here, MR damper installed between adjacent buildings is applied to seismic response control of two buildings. From review of the literature, a performance of groundhook control algorithm is superior and simple for reducing the vibration response of structure. In this paper, groundhook control model are applied to the multi degree of freedom system representative of buildings in order to reduce seismic response of adjacent buildings. It was demonstrated that MR damper is effective in controlling seismic response of two adjacent buildings simultaneously. And it has been shown that displacement response can be effectively controlled as adjacent buildings are connected at roof floors by MR damper. And acceleration response can be effectively reduced when adjacent buildings are connected at mid-stories by MR damper. Therefore, MR damper connected floor should be selected with control target as like displacement or acceleration.