The basic characteristics of ER fluid as Bingham plastic fluids in electric field were studied both experimentally and theoretically. In relation to the application of ER valve and ER damper, investig- ation on the damping force control was performed with the studies on the effects of electric fields, electrode geometry and oscillation flow. In the first experiment, using both the flat type and unevenness type electrode as ER valve of 2 mm height, the pressure drop was measured in two position to investigate the influence of the electric field strength and electrode type by the pressure transducers. The pressure drop and the shear stress are in proportional to the electric field strength. From the friction factor, ER effects are different by changing the flow and electric field condition. The in fluence of the yield shear stress did not appear with the increasing Reynolds number. In the second experiment, the possibility of damping force control of ER valve for the sinusoidal input by a piston cylinder is investigated. The ER damper model based on the Bingham plastic model are proposed, and the simulation results agreed well with the experimental results although the small deviations were shown. Therefore, ER fluid is considered as Bingham plastic model from the macroscopic view.