Purpose: This study was performed to evaluate transverse changes of proximal segment after bilateral sagittal split ramus osteotomy (BSSRO) using 3-D CT in mandibular prognathism. Patients and methods: Twenty-two patients who underwent BSSRO for mandibular set-back in class III malocclusion without facial asymmetry were examined. Miniplates were used for the fixation after BSSRO. Facial CT was taken before and after BSSRO within 3 months. Frontal-ramal inclination (FRI), inter-gonial width (IGW) and intercondylar width (ICW) were measured in 3-D CT images using V-works $4.0^{TM}$ program. Student t-test was used to compare the changes between pre- and post-operative measurements using SPSS 10.0 program. Results: 1. Mean peroperative FRI value ($12.87{pm}3.43^{circ}$) was slightly increased to postoperative value ($14.13{pm}3.72^{circ}$) (p < 0.05). The average FRI increased 10.42% and the range was from $-2.46^{circ}$ to $3.34^{circ}$. 2. Mean peroperative IGW ($99.01{pm}5.36$) was slightly decreased to postoperative IGW ($96.51{pm}5.88mm$) (p < 0.05). The average IGW decreased 2.52 % and the range was from $-6.61^{circ}$ to 0.91 mm. 3. Mean preoperative ICW ($125.01{pm}5.30mm$) was slightly decreased to postoperative ICW ($125.40{pm}5.45mm$) (p < 0.05). There is no significant difference between pre- and post-operative ICW. 4. There was significant correlationship between FRI difference and IGW difference (p < 0.05). Conclusions: These results indicate that the lower ramus of the proximal segment is moved inward after BSSRO procedure for mandibular set-back.