Background: The root canal system of the mandibular second molar is diverse and complex with many variations, especially C-shaped root canal system. Cone- beam computed tomography film is widely applied in dentistry and demonstrates its superiority in the application of root canal system research and endodontic treatment. Methods: The study was carried out in 25 patients with 32 mandibular second molars with C-shaped canals diagnosed with irreversible pulpitis who came for examination and treatment at the Department of General Dentistry, Odonto - Stomatology Center, Hue Central Hospital from May 2020 to May 2021. Study design: Descriptive cross- sectiomal study. Results: The most common site of caries was in the distal surface with 20 teeth, accounting for 62,6%. Among 20 distal caries, there were 15 caries caused by mandibular third molars (46,9%). The most frequent clinical symptom was pain, accounting for 75%. Teeth with 2 root canals are encountered with the highest rate, accounting for 50%. The most frequent configuration in the coronal third was C1, accounting for 65.6%. Of which, 2/21 teeth (9.5%) have 1 canal throughout the length, the rest split into 2, 3 canals in the middle and apical thirds. Conclusions: The most common C-shaped canal system in mandibular second molars were the C1 form. Conebeam CT film has many advantages in the application of root canal system research.