The aim of this study is to describe the endodontic treatment of a radix entomolaris (RE) with the C-shaped canal in mandibular first molar and a fused maxillary premolar with two supernumerary teeth with the separated root canal using cone-beam computed tomography imaging (CBCT). In our cases, non-surgical endodontic treatment based on the results of CBCT diagnosis, ProTaper rotary system, hand instrumentation, and passive ultrasonic irrigation along with periodontal treatment resulted in long-term and successful prognosis. Therefore, CBCT analysis an effective diagnostic tool for the treatment of teeth containing complex root canals.