In this paper, we propose the design of a novel gait rehabilitation passive orthosis to training the people who lost their walking ability due to neurological injuries, such as spinal cord injury and stroke. As known, the movements of pelvis play an important role in balance and propulsion during a gait cycle. In this research, we present and simulate a passive orthosis to assist the motion of pelvis of gait patients for regaining their initial walking pattern. This optimized pelvic device is un-motorized and contains only passive elements. The accomplished simulation is based on presentation of a proper 3D dynamic model of human body by focusing on motions of lower legs during gait on treadmill. Also, structural parameters of orthosis are derived during solution of optimization problem defined on related dynamic model. The simulation results show optimized time trajectories of pelvic rotations are close to corresponding desired diagrams and optimum passive device can create the desired motion of pelvis without any effort from subject.