Vehicle suspension is normally used to attenuate unwanted vibration from various road conditions. The successful suppression of the vibration leads to the improvement of ride comfort as well as steering stability. One of attractive candidates to formulate successful vehicle suspension is to use ER(electrorheological) fluid damper. This paper presents robust control performances of ER suspension system subjected to mass and time constant uncertainties. After identifying dynamic bandwidth of a cylindrical ER damper associated with two different ER fluids(one has fast response characteristic, while the other has slow response characteristic), a quarter car model is established by incorporating with time constant of the damping force. A robust H? controller, which compensates mass and time constant uncertainties, is designed in order to suppress unwanted vibration of the vehicle suspension. Control responses such as vertical acceleration are presented in time domain. In addition, the effect of time constant of the damping force on the vibration control performance is investigated via comparative work between fast and slow dynamic characteristics of the ER damper. The control responses presented in this work will provide very useful guidelines in adopting proper ER fluids for right application devices.