A prosthetic swing-phase control mechanism simulates the action of the upper leg musculature to aid in increased gait function. More specifically, swing-phase control mechanisms limit the maximum knee flexion and allow the shank to smoothly decelerate into full knee extension, without excessive impact. In this work, a magnetorheological (MR) damper is designed with the objective of controlling swing-phase damping in an above-knee prosthesis. A parametric model, the modified Bouc-Wen model, is used to represent the highly nonlinear dynamic properties of the MR damper. Based on this model, twelve control parameters that govern the hysteretic force and displacement of the damper have been identified. The parameters of the damper are determined through optimization of the prosthesis knee angle with a desired knee angle trajectory obtained from experimental data in normal level walking. Experimental data of thigh and hip motions are introduced as input into a dynamic system to find out a set of control parameters. A computer simulation is carried out. Comparison of the desired knee angle with the knee angle obtained from control parameters of the designed MR damper shows the effectiveness of the present design. Also, using the optimal control parameters, knee angle trajectories at zero and at lowered input currents, representing circumstances when the battery turns off and the power supply is reduced respectively, have been shown. Moreover, conditions of knee angle and shank velocity at the end of swing phase have been checked. The results obtained show a satisfactory performance of the system.