Sagittal split ramus osteotomy (SSRO) is widely used in treatment of dentofacial deformities. But, many complications can occur including unfavorable fractures during osteotomy. To prevent these complications, it is necessary to understand comprehensively the anatomy of the mandiular ramus. The purpose of this study was to evaluate the morphology of the madibular ramus in manibular prognathism patients by computed tomography comparing with normal control group. The study group consisted of 33 skeletal class III patients (20 males, 13 females) and the control group consisted of the 52 patients without dentofacial deformities (32 males, 20 females). The mean age of study group was 22.0-year old, and that of control group was 37.1-year. For the CT examination, following scan parameters was used: 1mm slice thickness, 0.5 second scan time, 120kV and 100mA/s. The axial scans of the head were made parallel to the mandibular occlusal plane. The anteroposterior length of the ramus, the distance from anterior border of the ramus to lingula, the relative distance from the anterior border of the ramus to lingula compared to the anteroposterior length of the ramus, the thickness of anterior and posterior cortical plate, the thickness of medial cortical plate of the ramus at lingula level, the thickness of cancellous bone of the ramus at lingula level were measured. The skeletal class III mandibular prognathism patients exhibited shorter anteroposterior length of the ramus, thicker anterior and posterior cortical plate, thinner mediolateral cancellous bone thickness. The lingula has a relative stable anteroposterior position in ramus in all groups. There was higher possibility of fusion of medial and lateral cortical plate at lingula level in the mandibular prognathism group. In conclusion, the mandibular prognathism patients have narrow rami with scanty cancellous bone, which means that careful preoperative examination including CT scan can prevent undesirable fractures during osteotomy.