Background: Mandibular contouring surgery is a common facial plastic surgery in Asia. The aim of this study was to evaluate the course of intraosseous inferior alveolar nerve within the mandibular canal in Thai population through three-dimensional computed tomography (CT) to find the safe surgical landmarks for mandibular contouring surgery. Materials and methods: This was a retrospective study in 200 patients who underwent computed tomographic assessment of head and neck region at the Faculty of Medicine, Ramathibodi Hospital, Mahidol University from January 2010 to December 2015. The inferior alveolar nerve was identified and traced along the mandibular canal from a three-dimensional image slice, and then the reconstruction of 3D images was performed. The oblique line, the alveolar arch line, the gonion, and O point were identified. Afterwards, the horizontal, oblique, and vertical distances were defined and measured. Comparison of these distances between each side was also analyzed. Results: The mandibular canal was far away from the posterior border of ramus for 17.58–17.62 mm in male, and 15.61–15.68 mm in female. It was far away from the gonion for 25.67–25.75 mm in male, and 22.73–22.77 mm in female. It was far away from the inferior border of mandibular body for 16.62–16.78 mm in male, and 15.75–15.84 mm in female. The statistical analysis showed no significant difference in all distances between each side in both male and female groups. Conclusion: A safe surgical landmark for angle mandible reduction is at a width less than 25 mm from the gonion in Thai men and 22 mm in Thai women.