Objective: To compare the straightening of root canal curvature, the apical transportation, the loss of working length and the working time using two rotary systems. Study design: Sixty buccal root canals with angles of curvature between 30 to 99º were instrumented with Mtwo and Twisted File rotary instruments. A series of pre- and post-operative radiographs were superimposed to determine the straightening of root canal curvature and apical transportation. The loss of working length and working time were also calculated. Results: All data were analyzed by means of the Student´s t-test and Mann-Whitney U-test. No statistically significant differences in the straightening of root canal curvature, apical transportation, or the loss of working length were found between groups. There was a significant difference in working time between angles of curvature when using the Mtwo system. Conclusion: Both rotary systems showed minimal changes in the original root canal curvature.