When treatment planning before placing dental implants in the posterior region of the mandible, the locations of the inferior alveolar nerve and mental foramen need to be ascertained, as they determine the bone height available and the implant length selected. The purpose of this study was to introduce the clinical application of a newly developed compact computed tomography system (Ortho-CT) to assess three-dimensional (3-D) images for the preoperative treatment planning of implants in the posterior region of the mandible. To evaluate the 3-D images, we scanned using the Ortho-CT system the mandible with a radiopaque template placed in the posterior region. The Ortho-CT images provided excellent information for evaluating the morphology of the mandible, and for showing the location of the inferior alveolar nerve, mental foramen and the relationship of the template to the bone. We consider that Ortho-CT is a useful aid to preoperative treatment planning of implant therapy in the mandible.