Introduction: During endodontic treatment the variety of mandibular sectorial in the form of an extra lingual (radix entomolaris) or buccal root (radix paramolaris) can often cause difficulties. In other words, awareness and understanding of this unusual root, and its canal morphology, are factors that can affect the outcome of root canal treatment. Case Presentation: A 30-year-old male patient with a history of severe, throbbing, constant pain in the lower mandibular molars was referred to the department of endodontics of Hamedan Dental university. The patient’s medical history was noncontributory. The buccal object rule (same-lingual opposite-buccal technique) confirmed the additional root as a distolingual root (radix entomolaris). Following the evaluation of vitality tests, we began endodontic treatment for this patient, after administration of local anesthesia using 2% lidocaine with 1:80,000 epinephrine (Daroupakhsh, Tehran, Iran) and rubber dam isolation. The working length was determined by a Root ZX apex locator (Dentaport ZX, J Morita) and later confirmed by parallel periapical radiograph. Canals were shaped in a crown down fashion with Protaper Nickel Titanium rotary instruments (Dentsply, Maillefer) under copious irrigation with 2.5% sodium hypochlorite and lubrication with RC-Prep. After drying the canals with paper points, the master gutta-percha points were fitted within the canals and confirmation radiography was taken. The root canal system was obturated with the cold lateral compaction technique. Conclusions: This case is about a mandibular molar with radix entomolaris and the radiographic exploration and endodontic order. Clinicians should be aware of these unusual root morphologies in the mandibular molars. The initial diagnosis of a radix entomolaris or paramolaris before root canal treatment is important to facilitate the endodontic procedure.