Combined mandibular angle resection with angle-splitting ostectomy (ASO) is more effective than conventional simple or multistaged ostectomy. Removal of the outer cortex of the mandibular body by ASO lessens the protuberance of the masseter muscle. In this study, the anatomy of the mandibular canals in seven human cadavers was studied in detail, and a guideline for ASO and mandibular angle ostectomy was set up so as to avoid injury to the inferior alveolar nerve. The most vulnerable area of the inferior alveolar nerve is the line from the gonion (G) to the junction (O) of the intersecting vertical line along the anterior border of the ramus and the horizontal line on the alveolar crest because of the thin anterior distance (AD) between the buccal surface of the mandible and the outer wall of the mandibular canal. The resection line should not be above 17.5 mm at the GO line to ensure a safe inferior distance, the distance between the inferior border of the mandible and the floor of the mandibular canal (21.6 +/- 4.1 mm). The body of the mandible was less vulnerable to injury to the inferior alveolar nerve in ASO because of the relatively thicker AD at the second molar (8.3 mm) and first molar (6.8 mm). Pilot surgery was performed in five cadavers. The lateral cortex was safely split off, avoiding injury to the inferior alveolar nerve, and angle ostectomy was then done. This method was applied in two clinical cases without any complications. The "split ostectomy of mandibular body and angle reduction" is a new and safe method of avoiding the injury to the inferior alveolar nerve.