Abstract: Objective To evaluate the sealing ability of Cshaped root canal system of mandibular second molars filled with GuttaFlow. Methods 63 mandibular second molars with Cshaped root canals were collected and then prepared with Hero shaper system.Then the teeth were randomly divided into 3 groups and the root canals were respectively filled with GuttaFlow,EQ plus thermoplasticized gutta percha and by cold lateral compaction technique.Then roots were sectioned at 2,4,6,8 mm from apical foramen.All sections were observed through stereomicroscope and the images were acquired.The percentage of guttaperchafilled area (PGP) was calculated with Image Analysis Software and the sealing ability was evaluated. Results At the 2 mm,4 mm levels from apex,sealing ability of GuttaFlow was better than thermoplasticized gutta percha and cold lateral compaction.At the 6 mm,8 mm levels,there was no significant difference between GuttaFlow and thermoplasticized gutta percha,but both were better than cold lateral compaction group . Conclusions GuttaFlow has good sealing ability in filling C shaped root canals of mandibular second molars.