The awareness of internal dental morphology is an excessively critical step in management of endodontic therapy. [1] The mandibular first molar, the earliest permanent tooth to erupt is assumed to be the tooth that mostly creates need for root canal treatment. The most common root canal morphology of the mandibular first molar is the presence of two canals in the mesial root and one or two canals in the distal root.[2] However, the literature indicates a number of case reports on the anatomic alterations and the variations related to the mandibular first molars. Amazingly, Reeh [3] reported a case with seven canals, involving four canals in the mesial root and three canals in the distal root.The knowledge of such anatomic variations of root and root canals is an essential aspect in managing the degree of success of root canal treatment. Without an efficient exploration, a canal could be left unprepared and inaccurate disinfection of root canal system is inevitable, so the endodontic therapy may result in failure. [4] This case report aims to present the management of a mandibular first molar with six root canals, four of which are placed in mesial and two of which are placed in distal root. Moreover, this case report highlights the usage of Cone Beam Computed Tomography (CBCT) as a diagnostic tool in endodontic practice.This case report aims to present the management of a mandibular first molar with six root canals, four in mesial and two in distal root. A 16-year-old male patient who has suffered from localized dull pain in his lower left posterior region for a long time was referred to the endodontic clinic. On clinical examination, neither caries lesion nor restoration was observed on the mandibular molar teeth; but the occlusal surface of the teeth had pathologic attrition. The mandibular and maxillary molars were tender to percussion due to bruxism, but there was no tenderness towards palpation. All of the molars revealed normal responses to the vitality tests. It was suggested that he should use the night-guard against bruxism. After three months, his pain almost completely relieved, but the percussion of the left mandibular molar was still going on. After access cavity preparation, careful examination of the pulp chamber floor with dental loupe and endodontic explorer (DG 16 probe) showed six canal orifices, four of mesially and two of distally. CBCT scan was performed in order to confirm the presence of six canals. Following one year, it was observed that he had no pain.