Electrorheological(ER) fluid is a kind of smart material with variable shear stress and dynamic viscosity under varying electric field intensity. The damping characteristics of ER damper can be controlled by applying electric field. The objective of this study is the analysis of the performance of ER damper and its application to shock absorber. Idealized nonlinear Bingham plastic shear flow model is used to predict the velocity profile between electrodes. The flow region is divided into three regions, inner post-yield region, central pre-yield region and outer post-yield region. Volume flux equations and velocity equations are solved simultaneously to obtain velocity profile and force applied to the piston. Cylindrical dashpot ER damper with moving electrode is constructed and tested under various electric fields. The analytic and experimental results about damping force are compared and discussed. Drop test system using ER damper is prepared to identify transient vibration characteristics. Without electric field, test system rebounded after initial impact. The rebound is eased as the applied electric field increases. When semi-active control algorithm is applied, rebound phenomenon disappear and vibration energy level decays faster than the case of zero electric field.