The electrorheological fluid(ERF)is a kind of intelligent material with bright prospects for industry applications, which has viscoelastic characteristic under the applied electric field. The dynamic model of a milling system with an ERF damper is established, and the chatter suppression mechanism of the ER effect is discussed theoretically. Both the theoretical study and the experimental investigation show that the additional damping and additional stiffness produced by the ERF increase with the rise in the strength of electric field E, but their influence on the cutting stability is different. Only when both additional damping and additional stiffness cooperate, the milling chatter can be suppressed quickly and effectively. In additional, an ERF damper used on the arbor of horizontal spindle milling machine is developed, and a series of milling chatter control experiments are performed. The experimental results show that the milling chatter can be suppressed effectively by using the ER damper.