Objective: To investigate the relationship between the inferior buccal branch and mandibular marginal branch of the facial nerve in the plane of angulus oris. Methods: Twenty unilateral adults cadaveric heads were dissected. In the vicinity of the posterior border of mandibular ramus, the positional relationship between mandibular marginal branch and the plane of angulus oris, the inferior buccal branch and the plane of angulus oris was recoded and analyzed. Results: In 18 of the 20 samples, the plane of angulus oris was between the inferior buccal branch and mandibular marginal branch in the vicinity of the posterior border of mandibular ramus. In one sample, the plane of angulus oris was below the inferior buccal branch and mandibular marginal branch in the vicinity of the posterior border of mandibular ramus. Another sample was excluded because the starting points of the inferior buccal branch and mandibular marginal branch were in front of the posterior border of mandibular ramus. The distance from the intersection of the posterior border of mandibular ramus and the plane of angulus oris to the intersection of inferior buccal branch and the plane of angulus oris was (14.96±8.55) mm. Conclusions In most cases studied, the plane of angulus oris is between the inferior buccal branch and mandibular marginal branch in the vicinity of the posterior border of mandibular ramus. Along the plane of angulus oris, within 1.0 cm anterior to the posterior border of mandibular ramus, it is a relatively safe place for surgical approach.