Abstract: Objective: The purpose of this study was to investigate the diagnosis with X-ray films and CBCT. It was to investigate radiographic features of the C-shaped canal system in mandibular second molars and their relationship to the canal anatomy. Methods: There were 110 extracted mandibular second molars for periodontal inflammation reasons in our hospital clinic patients. After taking their buccal –lingual radiograph. Each tooth was scanned with CBCT. Analyze the rate of diagnosing C-shaped root canal system and their types through preoperative radiographs and CBCT. Result: Among those 110 mandibular second molars, we coule diagnose 46 out through the preoperative radiographs and 66 through the CBCT. Each mandibular second molar with C-shaped canale diagnosed by CBCT was assigned to three categories : Type Ⅰ(Merging Type,26), Type Ⅱ(Symmetrical Type ,22), Type Ⅲ (Asymmetrical Type ,18).CBCT showed that in the type Ⅰcategory , C4 canal configurations were mostly found in the apical area. In type Ⅱ ,C3 were the main canal shapes over the whole canal length. In type Ⅲ, C1 and C2 were the main canal shapes in the coronal canal portion. Conclusion: These suggested that the mandibular second molars with C-shaped canal system have a series of radiographic characteristics, and it was possible to predict the presence and the configuration of most C-shaped canals by X-ray films. But if you want to learn detailed information, CBCT would be needed to use.