In this paper, a new decentralized structural active control algorithm using the instantaneous optimal control scheme is proposed. On the basis of finite element models, large-size structures are divided into smaller-size substructures. Each substructure is controlled by its own local controller; however, interconnection effect between adjacent substructures is taken into account by treating the interconnections as 'additional unknown disturbances' at substructural interfaces to the substructure. By sequential Kalman estimator for the substructural state vector and the recursive estimation of the 'additional unknown disturbances', instantaneous optimal control forces of each local controller are estimated without substructure interface measurements. Then, the proposed decentralized control algorithm is applied to the American Society of Civil Engineers 20-story benchmark control building under earthquake excitation and compared with the control criteria by the conventional centralized control approach. It is shown that the proposed decentralized control provides satisfactory control performances when compared with the conventional centralized control, but it increases the reliability of the control performance in case of the failure of local controllers. Robustness of the proposed decentralized control is also studied. Thus, the proposed decentralized control algorithm is suitable for structural active control of large-size structures.