Electro-rheological (ER) fluid is a kind of smart material with bright prospects for industrial applications. Its viscosity can be changed instantaneously by changing the intensity of an imposed electric field. In this paper two types of ER dampers, the MP one with multi-electrode plates and the SP one with sliding electrode plates, are designed. Experiments involving sinusoidal excitations and random excitations show that they are very effective in vibration control for excitation frequencies below 100 Hz, but are less or no longer effective for excitation frequencies over 100 Hz. It is the large viscous damping of the ER damper that suppresses the vibration in the lower frequency bands. However, ER fluid may change from liquid to a solid-like structure due to the high intensity of an imposed electric field, which causes the rapid rise of Coulomb frictions, but too large Coulomb friction will increase the response of the ER damper in the higher frequency bands. The MP damper has a broader varying range of damping than the SP one. Under high electric field intensity, the damping and stiffness of ER dampers change considerably, exhibiting strong non-linearity. The natural frequency of an ER damper shifts with the varying electric field due to changing stiffness. The viscous friction force and Coulomb friction force of an ER damper are estimated by a recursive least-square algorithm. The change in stiffness has been considered in the estimation. The estimated results agree very well with the experimental results.