Microscopic behavior of ER suspension structure between two fixed electrodes is visualized for the stationary and flowing ER fluids with high speed video camera fitted with a microscope under application of electric fields. The ER fluids used in the present study consist of cellulose suspended in silicone oil. Unipolar and bipolar sine-waves electric fields, and constant ones are used. The frequencies of the electric fields are from 0.1 to 1000 Hz. The pressure drop of ER fluids between two parallel-plate electrodes is also investigated under application of the electric fields for constant flow rates. Dependence of the pressure drop on electrical excitation frequency is investigated. Furthermore the flow characteristics of ER fluids which flow in an axial direction without the radial velocity component between two closely fitting concentric cylinders are examined. The pressure drop/flow rate performance of the ER fluids between concentric cylinders is measured under constant electric fields. The relationship between the pressure drop and the flow rate are calculated using the commonly quoted Bingham model.