Abstract: The properties of magnetorheological fluid (MRF) materials such as viscosity and shear modulus may vary quickly when the materials are subjected to varying magnetic fields. The MRF materials can be used to make the smart composite structures such as MRF sandwich beams and plates. In the paper, a finite element model of a simply supported MRF sandwich beam has been developed to investigate the vibration characteristics of the MRF sandwich beam structure with a MRF sandwich layer between two elastic aluminum layers. In modeling of the beam, the element matrices of three layers are formed, the dynamic equations are developed. The predicted natural frequencies and energy loss factors of the MRF beam have been validated by a performed experiment, indicating that the finite element results are agree well with the experimental results. The results show that the natural frequencies and energy loss factors of the MRF beam become higher with the increasing of applied magnetic field strength. The vibration of sandwich beam with MRF is of considerable suppression for applied magnetic field.