Magnetorheological (MR) fluid is a new kind of smart materials, which can be transformed from Newtonian fluid into visco-plastic solid rapidly in the presence of magnetic field. In this paper, the material composition, characteristics and constitutive relation of this smart material were discussed. The operational principles of MR devices in flow, shear and squeeze mode were generalized in detail. The design technique of structural parameters and magnetic circuit of MR devices were investigated. Especially, some important technical points on dimension of fluid gap, magnetic core material and the direction of magnetic flux density were sumarized. All of these can give a valuable referrence for MR devices design. As an example, a MR damper in shear-valve mode was designed, and some experiments were carried on it. From the experimental result, it shown that the MR damper has an excellent damping force characteristics. This also proved the design technique put forward by author was validated.