This is a report of 2-cases of mandibular prognathism corrected by Intraoral oblique splitting osteotomy of mandibular rami. The Intraoral oblique splitting osteotomy is a modification of sagittal split osteotomy of ramus and it is documented by Yoshida, on 1985. By this method, authors obtained the following results. 1. The patients' esthetic, psychological and functional problems were dissolved by setback of mandibular prognathism. 2. The postoperative infection splitted bone segments fracture, paresthesia of the face and T.M.J. dysfunction were not appeared. 3. Postoperative intermaxillary fixation was maintained for 8 weeks. The patients could open their mouths in normal range after a week of intermaxillary fixation removal. 4. The soft tissue changes of lower lip and chin were about 1:1 to the hard tissue changes. 5. During intermaxillary fixation period and postoperative orthodontic treatment, slight relapse was observed. Now, the patients are under postoperative orthodontic treatment.