Purpose: This study evaluated the pharyngeal airway space changes in CT images in patients receiving bilateral sagittal split osteotomy (BSSRO) for the surgical correction of mandibular prognathism. Methods: A total of 22 patients with mandibular prognathism were treated using BSSRO. Computed tomography was performed 1 month (T0) before surgery and, 1 month after surgery (T1). The anteroposterior length (AP), lateral width (LAT) and cross-sectional area (AREA) at the level of soft palate (C2) and base of the tongue (C3) were measured using CT images. Results: The mean amount of mandibular setback was 7.41 mm (${pm}$3.46 mm). All the AP, LAT and AREA at the C2 and C3 level were decreased significantly 1 month after surgery (P<0.001). As the amount of mandibular setback was increased, the AP, LAT and AREA levels at the level of C2 and C3 had decreased. In addition, the reduction of the AREA at the C3 level was associated with the amount of mandibular setback (P<0.05). Conclusion: A significant decrease in pharyngeal airway space was observed 1 month after the operation. The cross-sectional area at the level of base of tongue was decreased with increasing amount of mandibular setback.