Abstract: The algorithms of semi-active control for application of MR damper to seismically excited building structures are studied in the paper. The restoring model of MR damper is introduced and the parameters of the damper are designed. Semi-active H_∞ control laws based on full-state feedback and observers are proposed. A numerical simulation is made on the seismic responses of a six-story building frame structure with two MR dampers. The results show that the semi-active H_∞ control using MR damper can reduce the structural seismic response efficiently and have good robustness.