Abstract: Magnetorheological (MR) damper is a new kind of smart damper, which has great potential in semi-active control of mechanical vibration. A flow mode MR damper was designed and manufactured. The paper discusses qualitative behavior of the MR damper from the experimental data when the damper was subjected to sinusoidal excitation. A nonlinear hysteretic bi-viscous model to describe the performance of the MR damper is suggested. Afterwards, the parametric identification of the MR damper is outlined. The relationships between viscous coefficient, applied current and activating frequency are investigated. In addition, the operational effects on the parameters of the model are investigated.