27 patients with mandibular prognathism were studied with 40 normal people as controls. All subjects were examined with electromyogram. The result shows that the voltage were lower in patients before surgery, the double-silent period inspect rate were much higher than normal subjects. It was concluded that the electromyo-activity of masticatory muscles had some character in patients after surgery: (1) The voltage of masseter and temporal muscle increased significantly during maximal voluntary clenching. (2) During protrusive border movement the voltage of masseter increased while the changes of voltage in the temporal muscle were small. (3) In lateral border movement, the voltage of masseter and temporal muscle increased in the working side, while the balaning side changed slightly. (4) Electromyo-voltage decreased in rest position. (5) The duration of silent period had a tendency of shortening after surgery. It demonstrated the feasibility and necessity of treating mandible deformity through orthognathic surgery, and it provided theoretical and practical support of evaluating the treatment effect of such kind of patients.