The effectiveness of a semi-active control method using magnetorheological (MR) dampers on reducing the deck displacement of the isolated bridges, which exhibit nonlinear responses at both the columns and isolators under near-field ground motions, is studied. The sliding mode control algorithm is utilized to command the MR dampers. A five-span viaduct with high-damping-rubber bearings is analyzed to evaluate the control performance. The results show that the semiactive control using MR dampers is capable of providing the similar performance by the active control. The semi-active control based on the sliding mode control algorithm achieves outstanding control performance through comparison with the semi-active control based on the LQR control algorithm.