Objective:To report a case of germinated teeth of the left mandibular second molar diagnosed by cone-beam CT (CBCT), and to investigate its differential diagnosis, pathogenesis, and the application value of CBCT in the diagnosis of oral and maxillofacial diseases.Methods:One case of germinated teeth of the left mandibular second molar diagnosed according to oral general examination ad CBCT findings in Zhenjiang Stomatological Hospital, China in March, 2019 was included in this study. The characteristics and differential points of supernumerary cusp, fused teeth, geminated teeth and concresence of teeth were analyzed based on literatures.Results:CBCT examination showed that the number of mandibular dentition teeth was normal, there was a large area of low-density transmission area around the root of the left mandibular second molar. Three-dimensional reconstruction results revealed that the suspected supernumerary teeth were fused with the root of the left mandibular second molar. Sagittal projections showed that the suspected supernumerary teeth were located in the buccal side of the left mandibular second molar. Axial projections showed three separate root canal orifices at the level of pulp chamber floor, and the dentin of the two was connected. The suspected supernumerary teeth had an independent pulp cavity and a clear root canal, and fused with the distal root canal of the left mandibular second molar in the middle of the root to form a root canal. A "Y"-shaped structure was displayed on the sagittal projections. CBCT showed that the left mandibular second molar was a fused root. Based on oral clinical examination, the left mandibular second molar was confirmed to be a germinated tooth.Conclusion:CBCT is one of the most important means of oral auxiliary examination. It has significant advantages in the diagnosis of tooth abnormalities. It can help clinicians to make correct diagnosis and choose the appropriate treatment scheme. It has certain clinical significance and innovation.