To compare ProTaper Universal (PU) and M3-L instruments in preparing curved root canals and to provide a reference for clinical applications.

Twenty resin-simulated curved single root canals were randomly divided into two groups. Root canal preparation was performed with a PU nickel-titanium file or an M3-L nickel-titanium file. Root canal preparation time, root canal length before and after preparation and root canal deviation were recorded.

In the M3-L group, the displacement measured at the 4th to 7th sites was less than that in the PU group (P < 0.05), but there was no significant difference between the other two groups (P > 0.05); The length of the root canal before and after root canal preparation in the M3-L group was (14.90 ± 1.92) mm and (14.57 ± 1.13) mm, respectively, and the difference between the two groups was not statistically significant (t＝3.18, P＝0.058). The root canal length before and after root canal preparation in the PU group was (14.53 ± 1.53) mm and (14.28 ± 1.39) mm, respectively. The difference between the two groups was not statistically significant (t＝2.12, P＝0.124); The average preparation time of the M3-L group [(110.15 ± 10.43) s] was less than that in the PU group [(330.48 ± 12.62) s] (P < 0.05).

The M3-L nickel-titanium file has better central positioning ability and is less time-consuming than the PU nickel-titanium file in root canal bending preparation.