Electrorheological (ER) materials experience real-time and reversible changes in rheological and mechanical properties when subjected to electrical fields, which draw much attention to the use of ER materials in adaptive structures recently. ER sandwich beam is composed of two elastic layers and sandwiched ER layer. By controlling the pre-yield rheological and mechanical properties of ER materials, the dynamical response of the composed beam is then controlled. In this paper, ER sandwich beam simply supported at two ends is moded. The dynamical response of ER sandwich beam and the optimal electrical fields corresponding to different frequency are analyzed.