In this present research work, a new configuration of the hybrid magnetorheological (MR) brake via T-shaped drum with an arc form surface boundary - the biomechanical geometric design of the hybrid MR brake as a prosthetic knee - is discussed and experimentally tested. The main purpose of this study is to develop a prosthetic knee with one rotary disc to fulfill the desired objective. To achieve this, three steps are considered. In the first step, to model the brake, slab method modeling is used to calculate the braking torque due to the arc surface. In the second step, the biomechanical geometric design is adjusted as an optimization problem to maximize the braking torque, minimize the instantaneous electrical power dissipation and maximize torque density via three biomechanical constraints. In the third step, the proposed prosthetic knee is verified through simulation and experiment. Two evaluation criteria namely, controllability and efficiency are considered for the fabricated MR brakes. The results demonstrate that the optimized MR brake with 0.69-kg weight generates 38.5-N·m braking torque with more reactivity. © 1996-2012 IEEE.