Abstract: Magnetorheological (MR) fluid damper is a relatively new type of device for application to semi-active vibration control.The essential characteristic of the fluid is its ability to reversibly change from a free-flowing,linear viscous fluid to semisolid with a controllable yield strength in milliseconds when exposed to an magnetic field.External energy required by the adjustable fluid damper is minuscule and the damping force produced by the damper is large,while speed of its response is on the order of milliseconds.In this paper,the performance and equivalent linear damping coefficient of MR damper are analyzed based on the Bouc-Wen model,the energy dissipation method and the trapezoidal rule.Applying the MR damper model to an oscillator with signal degree of freedom harmonically excited through its support,the transmissibility which is the degree of relative isolation and defined as the ratio of amplitude of motion of the oscillator to amplitude of motion of the support is determined,and the phase angle of steady response as well.