Abstract: In this paper the Electrorheological(ER) fluid and its characteristics are introduced briefly and the application of an ER damper in rotor vibration control is studied experimentally.It is found that by increasing the applied electrical field strength,the damping and stiffness of ER damper are increased, the vibration amplitude of the rotor system supported on ER damper is decreased in the sub-critical speed range and at the critical speed, and is increased in the super-critical speed range. The critical speed of the rotor system can be changed by means of the applied electrical field strength. The damping provided by the ER damper has the properties of both the Coulomb damping and the viscous damping. Experimental results show that the ER damper has obvious effect on vibration control,and features instantaneous control,little energy consumption and broad frequency range.Besides,it's structure is simple.