Abstract: The application of full-information Η_∞ control theory to vibration attenuation of buildings with control time-delays under seismic excitation was presented. By introducing the extended vectors in state space, the motion equation of original system with time-delays was transformed into the standard discrete form that contains no delays. Then by using balanced-reduction method for this discrete system, the dimension can be significantly reduced. Thusly, a robust controller was designed based on Η_∞ disturbance attenuation theory, concerning the uncertainty of seismic excitation. The vibration-reduction controller contains linear combination of some former steps of control, which reflects the character of controlled systems with time-delays. It is also suitable for the case of large time-delays. Simulation results for a three-story building demonstrate the effectiveness of the control method presented. Furthermore, the parameter effects on the performance of the controller are also discussed. Moreover, the dimension of the controller can be reduced without discounting the control efficiency.