A design optimization methodology is presented to maximize the modal damping factor of a partially treated MR sandwich beam. Modal strain energy approach has been implemented in the finite element model developed for a partially treated MR sandwich beam to derive the modal damping factor at each node. Two different configurations of a partially treated MR sandwich beam are considered including a beam with a cluster of MR fluid segments and a beam with arbitrarily located MR fluid segments. The significance of the location of MR fluid segments on the modal damping factor is investigated under different end conditions. The design optimization problem is formulated by combining the finite element method with optimization based on genetic algorithm (GA) to identify the optimal location of the MR fluid segments to maximize the modal damping factor for the first five modes individually under various end conditions. The results suggest that the optimal location of the MR fluid treatment is strongly related not only to the end conditions but also to the mode of vibration. Copyright © 2010 by ASME.