A magnetorheological (MR) fan clutch is introduced. Based on Herschel-Bulkley model, the equation of transmission torque developed by MR fluids is derived to compute the torque transmission ability in the MR fan clutch. The necessary gap and the need of MR fluid volume between two parallel circular discs of the clutch are carried out. The results indicate that, the transmission torque of the MR fan clutch developed rapidly depend on the yield strength of MR fluid. The necessary gap and the volume for the MR fan clutch can be obtained based on MR fluid properties, the desired control torque ratio, the angular velocity and load torque of the clutch.