In the active control of structural response, the active mass damper installed in the top floor has been recognized to the most promising device for vibration control of high-rise buildings subjected to wind load and earthquake. For the design of active vibration control system, it is essential and unavoidable to derive a reduced-order model of the structure, which may invite a spillover instability problem. This study describes a new methodology using the reduced-order model to design the controller. The presented study, an active mass damper based on the Ac servo motor for the suppression of sway motion of structures, is investigated both theoretically and experimentally. In addition, the experimental model of a six-story structure with low fundamental frequency was constructed elaborately to comprehend the dynamic behavior of the flexible structure. According to the proposed control algorithm, the results of experiment indicate a very pronounced decrease in structural response and show both feasibility and possibility of the active mass damper to the tall buildings.