The aim of this study was to elucidate the anatomical structures that are vulnerable to injury during sagittal split ramus osteotomy (SSRO). Twenty-nine hemifaces of 19 Korean adult cadavers (11 men and 8 women; age range, 50-91 years) were dissected, and the locations of the facial nerve, retromandibular vein (RMV), and external carotid artery (ECA) were measured on the base of the mandibular posterior border and occlusal plane. Sagittal split ramus osteotomy was performed on the cadaver heads at intervals of 10 mm, and the proximity of the facial nerve was observed. The buccal branch and mandibular branch crossed the posterior border of the mandible (PBM). Most buccal branches (86%) crossed between 7/10 and 10/10 of the distance from gonion to mandibular notch (MN). Most mandibular branches (86%) were between 6/10 and 1/10 of the distance from gonion to MN. Most facial nerve trunks (FNTs) (82%) were within a circle of 9 mm in radius. Its center was located 34 mm posterior and 7 mm inferior to the MN. The FNT was located in the range of 11 to 14 mm medial to the PBM. The FNT emerged out of the stylomastoid foramen and ran anteroinferiorly in a direction of 45 degrees. In 10-mm ramus setback osteotomy, FNT was very close to the PBM, running almost vertically. Retromandibular vein was 5.5 to 8.6 mm posterior and 4.2 to 9.1 mm medial to the PBM. The ECA was located at 5.7 to 6.5 mm posterior and 10.5 to 12.9 mm medial to PBM. Facial nerve could be averted from injury by doing less setback. Bleeding after SSRO is likely to be due to the injury of RMV which is closer to the PBM (4-9 mm) than ECA (12-13 mm).