In this study, the frequency response of rectangular sandwich plates with multi-layer face sheets and electrorheological (ER) fluid cores is investigat-ed. The assumed electro-rheological fluid as a core is capable of changing the stiffness and damping of structures. To model the sandwich panel, for the first time, first-order shear deformation theory and the second Frost-ig's model are applied for the face sheets and thick cores, respectively. The studied sandwich panel is supposed to simply support boundary in all edg-es, and the Galerkin approach is implemented for discretizing the problem. In the result section, impacts of various parameters such as electric field, aspect ratio, the thickness of the ER layer, and thickness ratio on vibra-tional characteristics of the structure are discussed in detail. The obtained results highlight the notable effects of the electric field on natural frequen-cies which can make the structure flexible within the desired range. It is al-so pointed out that the dynamic behavior and stability of the system can be controlled by changing the magnitude of the ER fluid layer.
