Objective: To evaluate the treatment efficacy of reverse flaring technique applied by two rotary instruments, Hero 642 and ProTaper, on complicated molar root canal preparation. Methods: 100 molars with curved root canals (maximum curved angle ≥25 degrees) suffered from pulpitis or periapicities were randomly divided into two groups. In experimental group reverse flaring technique was applied by rotary instruments while in control group reverse flaring technique was not adopted. 50 experimental teeth with curved canals were operated with K files, Hero 642 and ProTaper in sequence. Lateral condensation obturation method was utilized in both groups. Root canal preparation and obturation efficiency were evaluated by X-ray, root canal preparation time and complication incidence (1 year follow-up rate was 95%). Results: Reverse flaring technique applied by Hero 642 and ProTaper Ni-Ti rotary instruments on complicated (curved) root canal preparation in group A demonstrated better root canal coning and smoothness, without instrument fractures, while instrument fractures occurred in control group. Significant difference was found in exact root canal obturation rate and not enough full rate, root canal smoothness and postoperative pain between two groups(P<0.05). Conclusion: Reverse flaring technique applied by Hero 642 and ProTaper NI-Ti rotary instruments indicates complementary potencies, demonstrating satisfactory root canal shape and obturation effectiveness, and lower complication incidence. Reverse flaring technique applied by Ni-Ti rotary instruments is apt for medium/severe curved root canal preparation and worthy of clinical application.