Deep brain stimulation on the subthalamic nucleus has been used to relieve Parkinsonian motor symptoms. However, the underlying physiological mechanism has not been fully understood. Beta-band cortico-muscular coherence increases when healthy humans perform isometric contraction. We hypothesized that this might be a measure of symptomatic improvement in motor performance after subthalamic nucleus deep brain stimulation. Here, we measured the beta-band cortico-muscular coherence with magnetoencephalography from three Parkinson's disease patients. We then compared the coherence values for stimulator on-state and off-state. We found that when the stimulator is on, the beta cortico-muscular coherence elevates significantly for the tremorous hand compared with that when the stimulator is off. This suggests that deep brain stimulation resulted in better cortico-muscular coordination.