Structural design of a kind of cylindrical magneto-rheological fluid（MRF） clutch is studied theoretically. Bingham model is used to characterize the constitutive equation of MRF subjected to an external magnetic field.Working principle of the MRF clutch is analyzed. The computational model of the transmitted torque and the rotational speed output of the clutch is set up and the expressions for clutch design were derived.Based on these expressions the structural parameters of two kinds of cylindrical MRF clutches are calculated with two kinds of chosen MRF. Study results show that the transmitted torque and the rotational speed output of the clutch can be adjusted and controlled by altering the magnetic field strength. The performance of the chosen MRF can make crucial influence on the structural parameters of the clutch.