Electro-Rheological (ER) fluid is a smart material. It develops a new path for automotive semi-active intelligent suspension system. The damping force of ER damper can be controlled by applied electric field. Applying the theory of rheodynamics and hydraulic theory, a relationship of damping vs. vibration velocity, electric field strength and gas-filled pressure has been set up. There are three sections of damping force, background damping force of a fluid viscosity, electric damping force of applied electric field and pressure of gas. A new gas-filled ER damper is developed. The main structure parameter of influencing ER damper performance are discussed, some design principles of ER damper are given.