Abstract: Objective To investigate the clinical efficiency of cone-beam computed tomography (CBCT) in the diagnosis and treatment of C-shaped canals of mandibular second molars. Methods Sixty-two mandibular second molars that failed in the root canal therapy were included in this study. The periapical films could not indicate the anatomic forms of root canals and the suspicious missed canals. Results Fixty-two canals (83.87%) were diagnosed as C-shaped canals, among which the numbers of C1 type, C2 type and C3 type were 28 (53.84%), 18 (34.62%) and 6 (11.54%), respectively. Among the 62 teeth with C-shaped canals, 2 teeth failed to receive the root canal therapy due to root canal perforation, and 60 teeth received gutta-percha filling through warm gutta-percha obturation and vertical condensation. After endodontic retreatment, 60 teeth (96.77%) showed satisfactory outcome with no complications identified. CBCT functioned in positioning the lesioned root canals with a high accuracy. Conclusions CBCT can analyze the configuration of root canals in a three-dimensional view, which is of prime importance in the diagnosis and treatment of C-shaped canals of mandibular second molars.