BACKGROUND:Severely curved root canal preparation is the difficulty during clinical root canal preparation, and improper preparation may induce complications such as stage, root canal offset and apical open, which affect the quality of root canal preparation and apical closure. Therefore, it is important to choose a root canal preparation instrument which has the best cutting and forming ability. OBJECTIVE: To compare the shaping ability of Mtwo nickel-titanium rotary instruments, K3 nickel-titanium rotary instruments and stainless steel K-files for the preparation of curved root canals. METHODS:A total of 30 severely curved root canals of freshly extracted human maxilary molars were randomly divided into three groups, with 10 canals in each group. The canals were prepared by Mtwo instruments, K3 instruments and K-files, respectively. The instrument’s tip location in pre- and post-instrumentation radiographs of the three groups was recorded; the changes of root curvature, working length and working time in the three groups were measured and compared. RESULTS AND CONCLUSION:The root curvature showed no significant difference in the three groups before preparation (P> 0.05). The changes of root curvature after root preparation in Mtwo instrument and K3 instrument groups were significantly less than that in K-files group (P < 0.01), and no significant difference was found between Mtwo instrument group and K3 instrument group (P > 0.05). The root canal preparation time in the three groups showed significant differences (P < 0.01), which was the shortest in Mtwo instrument group and the longest in K-files group. The changes of working length after root preparation in the three groups showed no significant difference (P > 0.05). Mtwo nickel-titanium rotary instruments and K3 nickel-titanium rotary instruments are have high efficacy and strong shaping ability in preparing curved root canals.