The purpose of this study was to examine how the soft tissues were changed and adjusted by 3-D CT analyzing masseter muscle of the patients who had skeletal Class III malocclusion with facial asymmetry in one year and about four years after bimaxillary orthognathic surgery. This study was also to observe how the surgery impacted the masseter muscle in the long term by comparing with normal occlusion. Eight patients who were diagnosed with skeletal facial asymmetry and had the surgery were selected as a asymmetry group, and ten subjects who have normal jaw and normal occlusion were selected as a normal group. 1.When the mandible one year and about four years after the surgery in the asymmetry group were measured, the gonial angles in the shifted side (p<0.05) and in the non-shifted side (p<0.01) both significantly increased. 2.The changes in the measurement of mandible mostly occurred within a year after mandibular setback osteotomy, and after one year there were hardly any changes.3.The most related variable to the volume of the masseter muscle about four years after mandibular setback osteotomy came out to be the maximum cross sectional area of the masseter muscle and the thickness of the maximum cross sectional area (p<0.01).4.Before mandibular setback osteotomy, there were significant differences between two groups in volume, masseter muscle angle, maximum cross sectional area, thickness of the maximum cross sectional area, width and length of the masseter muscle. However, about four years after the surgery there were no significant differences.