Abstract: Objective To evaluate the efficacy of GuttaFlow in C-shaped canals in mandibular second molars. Methods Select 80 teeth in 76 cases of C-shaped canals in mandibular second molars that needed treatment. The teeth were randomly divided into two groups, 40 in each. Root canals in the experimental group were filled with GuttaFlow and gutta-percha points. Root canals in the control group were filled with cortisomol and dealed with cold lateral condensation of gutta-percha. Compare the filling time, quality of root canal filling according to X-ray photos, pain within one week after the treatment, and curative effects one year after the treatment. Results The filling time of experimental group was obviously shorter than that of the control group. The occurrence of postoperative pain was low. The quality of root canal filling and curative effects were better than those of the control group. The difference was significant between two groups（P<0.05). Conclusion GuttaFlow is easy to operate in C-shaped canals in mandibular second molars. It's effective and worth popularization and application in clinics