The purpose of this study was to investigate the relation between the radiographic features of C-shaped mandibular second molars and the cross-sectional shapes of their root canal system. Fifty-eight extracted mandibular second molars with fused roots were collected from a native Chinese population. The teeth were scanned at 0.5-mm interval thickness by micro-computed tomography (microCT) and observed at 11 levels. The 54 teeth that possessed a C-shaped canal system were further examined. Their radiographic appearances were classified into three types according to discernible radiographic features. The results were subject to the Kruskal-Wallis statistical test. In these 54 molars, 16 teeth (29.63%) showed a type I radiographic image, 22 (40.74%) type II, and 16 (29.63%) type III. In the type I category, the C1 and C4 canal configurations were mostly found in the apical area. Categories C2 and C3 were the main configurations in the middle and apical areas in type II and III. The type III had more C2 canals in middle area than type II. These results suggested that it was possible to predict the presence and the configuration of C-shaped canal system by the radiographic appearance.