PURPOSE: To determine the anatomical characteristic of C-shaped canal systems in Korean mandibular 2nd molars by cone beam computed tomography (CBCT). MATERIALS AND METHODS: The 824 CBCT images were evaluated. These patients were taken CBCT for the diagnosis and treatment of dental implantation on 2013. The 711 mandibular 2nd molars were examined. The configuration of root canal systems were classified according to modified Melton's classifications. RESULTS: Of the 711 mandibular 2nd molars, 21.5% had C-shaped canal systems. This prevalence did not differ with tooth position. Most of the C-shaped canals with bilateral molars were symmetrical. Of the C-shaped canal, the most common configuration Type were Melton's Type I (89%) in the orifice level and Melton's III (83.8%) in the apical level. The prevalence of C-shaped canal was higher in female (25%) than male (13.9%) (P < 0.05). CONCLUSION: A high prevalence of C-shaped canals were observed in Korean mandibular 2nd molars. For successful C-shaped root canal treatment, the comprehension of root canal systems was important.