This paper presents vibration characteristics of magnetorheological(MR) elastomer, whose elastic modulus are controllable by applied magnetic field. By using this property, the material can be applied to vibration absorber, so that the stiffness of the absorber can be changed and actively controlled according to the magnetic flux density. However, the various performances of MR elastomer depends on different polarized direction of particles by applied magnetic field and dimension during the manufacturing process. In this paper, in order to obtain the optimal characteristics of MR elastomer, MR elastomers with different types and dimensions are prepared for a series tests. Using this test setup, extent of natural frequency shifted against magnetic field at various excitation frequencies can be measured. Specimens are prepared with 3 types, as cylinder samples exposed to magnetic field vertically, horizontally and unexposed during cure, respectively. Also, a set of design variables are considered to produce MR elastomers. Through the modal tests of mass structure with MR elastomer, the optimal design as well as the polarization direction of MR elastomer is obtained among the various dimensions and 3 directional types of MR elastomers.