This study presents the development of a MR damper applied to the suspension of tracked vehicles to improve the ride comfort and mobility. The MR valve is constrained to a specific cylindrical volume in order to keep the same geometry with the original damper. Radial duct is selected for its high efficiency and small volume requirement. The design rule is to enlarge the dynamic force range of the damper under the condition that the off-state force accords with the fail-safe principle. So the mechanical modeling is established and the influencing factors of adjustable coefficient are analyzed so as to determine the geometry parameters of MR valve. A set of experiments are performed to verify the theoritical analysis. The experimental results illustrate the designed MR damper has a good performance of energy dissipation and can be used to practical applications.