Aiming　at　the　semi-active　control　of　the　machining　chatter　in　boring,　a　new　type　boring　bar　is　designed,　which　contains　an　electrorheological　(ER)　fluid　and　is　with　a　controllable　vibration　characteristic.　An　investigation　to　the　nonlinear　dynamics　of　the　ER　fluid　in　the　bar　is　given,　which　is　dependent　on　the　applied　electrical　field　strength　and　vibration　amplitude.　The　experimental　and　the　theoretical　analysis　are　presented　to　prove　that　the　ER　fluid　in　the　bar　serves　as　the　friction　damper　with　on-line　adjustable　parameters　under　some　certain　conditions.　Lastly,　an　experimental　result　about　the　on-line　chatter　controlling　based　on　the　nonlinear　dynamics　of　ER　fluids　is　given.