According to the recent development of robot technology, quite big interest arises in research of actuating device for a joint of a manipulator. In this study, a compact magneto-rheological (MR) fluid brake with multi-layered disks is designed that can be used for such purpose. Viscosity variation of MR fluid is formulated, and torque is calculated by counting both magnetic effect and fluid viscosity. The rotor is designed with multi-disks to increase surface area to get more brake torque. The gap between the stator and the rotor is properly obtained with considering magnetic reluctance of the MR fluid. The estimated torque of the designed MR brake is calculated with analytic formulation. The rotor disk size is determined that the MR brake produces a desired torque of 10 Nm.