The effectiveness of seismic displacement response control for isolated bridges, which exhibit nonlinearity at both the columns and the isolators, is studied using the LQR optimal active control. A typical viaduct is analyzed for evaluation. A parametric study of weighting matrices is carried out to examine the effect on reducing the deck displacement response. The results indicate that the LQR active control is effective in reducing the deck displacement at small control force levels. However, the larger control force is not beneficial to reduce the deck displacement because it causes large column post-yielding displacement. The effectiveness of saturation of control force is thus investigated for preventing excessive control at large control force levels. Finally, the active control is compared with the passive control using viscous dampers.