Peripheral facial nerve paralysis appeared in 4 of about 250 cases following sagittal splitting osteotomy of the mandibular ramus at the Department of Orthodontics, School of Dentistry, Iwate Medical University, during the past 27 years. The operation consisted of Obwegeser-Dal Pont’s osteotomy, Epker’s modification and Obwegeser’s osteotomy technique. Causes of peripheral facial nerve paralysis during the postoperative course were analyzed and determined on the basis of the location of the lesion and its symptoms, the method of operation, and the CT-Scanning findings. The following results were obtained: The complication in the first case was presumably attributable to the compression of the peripheral facial nerve due to the postoperative hematoma, as assessed by CT-Scanning two months after operation. The second case was suspected to have resulted from viral infection because the appearance of peripheral facial nerve paralysis became clearly perceptible 27 days after the operation. The third case was possibly due to the compression by postoperative edema or viral infection. The fourth case was perhaps due to direct surgical damage to the nerve.