This paper presents an effective design strategy for a magnetorheological (MR) damper using a nonlinear flow model. The MR valve inside a flow mode MR damper is approximated by a rectangular duct and its governing equation of motion is derived based on a nonlinear flow model to describe a laminar or turbulent flow behavior. Useful nondimensional variables such as, Bingham number, Reynolds number, and dynamic (controllable) range are theoretically constructed on the basis of the nonlinear model, so as to assess damping performance of the MR damper over a wide operating range of shear rates. First, the overall damping characteristics of the MR damper are evaluated through computer simulation and, second, the effects of important design parameters on damping performance of the MR damper are investigated. Finally, the effective design procedure to meet a certain performance requirement is proposed. A high force-high velocity damper is fabricated and tested, and the resulting model and design procedure are experimentally validated.