Objective:The aim of the present study was to evaluate maintenance of the original anatomy of the root canal diameters instrumented with the Mtwo rotary instruments and BioRace file systems. Material and method:Acrylic resin blocks with 160 simulated root canals with an angle curvature 15°-30° were taken into consideration in the present comparative study. The 160 simulated root canals were divided into two groups, each of them containing 80 acrylic resin blocks. Each group was divided into two subgroups (n=40 canals each). The simulated root canals subgroups were prepared with Mtwo and BioRace rotary nickel-titanium instruments. The results were scrutinized using analysis of variance (ANOVA)-test and Tukey's test. The significance level was set at P < 0.05. Results:No significant difference was noted between the Mtwo rotary instruments and BioRace files as they both had equal value in the coronal section (P=.000) in S1 (D1) and S1 (Dpi). The results of Tukey's test revealed a significant difference between the Mtwo rotary instruments and BioRace files in middle and apical section with the value (P=.003) in S3 (Dpi) and S4 (Dpi). The results of Tukey's test found out a significant difference between the Mtwo rotary instruments and BioRace files in the middle and apical section with the respective values (P=.003) in S3 (Ai) and (P=.006) in S4 (Ai). Conclusion:Ccording to the findings of the present study, enlargement of the root canal diameters resulted to be better on the middle and apical section of the canal prepared with Mtwo instruments in comparison with BioRace instruments. Thus, Mtwo instruments will increase the rate of success of endodontic therapy.