Objective:To evaluate the root and canal morphology of mandibular first molars in Chinese individuals using cone-beam computed tomography(CBCT). Methods:A total of 766 patients of Chinese descent with healthy,well-developed mandibular first molars were included. These patients went to the Stomatological Hospital Affiliated of Nanjing Medical University and underwent radiographic examination by CBCT as part of their routine examination,diagnosis and treatment planning. The root canal configurations were classified according to the method of Vertucci. Results:CBCT image data of 1,395 mandibular first molars from 766 patients (137 patients without unilateral mandibular first molars) were obtained. The prevalence of mandibular first molars with two separate roots was 68.5%,however,three roots were identified in 31.4% of mandibular first molars. Three canals were found in 57.9% of mandibular first molars and 38.5% had four canals. 93.1% mesial roots contained two canals,most distal roots had a simple type I configuration,whereas mesial roots had more complex canal systems. The incidence of permanent three-rooted mandibular first molars was 35.3%,the incidence had no significantly difference between male and female (P > 0.05). Of all the patients with 3-rooted mandibular first permanent molars,60% occurred bilaterally. There was a significantly higher incidence of three-rooted mandibular first permanent molars on the right side than on the left (P > 0.05). Conclusion:Three-rooted mandibular first molars occurred frequently in this Chinese population. CBCT is an effective tool for the detection of additional distolingual roots and it is a valuable aid for dentists providing root canal treatment.