The inferior alveolar nerve and its branches are critical when performing oral surgical procedures in the posterior mandibular area. Any injury to the nerve bundles may lead to ineffectiveness of local anesthesia, paresthesia and numbness. The purposes of this study were to investigate the prevalence, location, and dimension of accessory mental foramen (AMF) in a Taiwanese population. Cone-beam computed tomography (CBCT) images of 100 patients were analyzed to identify AMFs. AMFs were observed in 6% of patients, including 3 males and 3 females (mean age: 55.0 ± 9.7 years). All AMFs were unilaterally located between the second premolar and the first molar. The mean diameter of the AMFs was 1.4 ± 0.2 mm. Therefore, clinicians should be aware of anatomical structures such as AMFs during surgical procedures over the posterior mandibular region. CBCT is a useful pre-surgical assessment tool for forming appropriate treatment plans and reducing neurovascular complications.