In this case study, we investigated the effects of robot-assisted gait training on the plasticity of motor system in a stroke patient using functional MRI. A patient who suffered from the left hemiparesis following the right MCA infarction performed gait training with a robot-assisted gait orthosis. Before and after gait training, motor performances were assessed and functional MRIs were acquired with motor activation task of affected lower limb. After gait training with a robot-assisted orthosis, the patient's motor performances were improved and cortical activities were changed. Activation in the ipsilesional primary sensorimotor cortex was increased and cortical reorganization was induced in a way that nearby regions were recruited for the movement of affected lower limb. The results of this study showed that gait training with a robot-assisted orthosis induced cortical reorganization of the motor network that resulted in enhancement of motor performance of the lower limb.