The aim of this study was to determine the shaping ability of Mity Roto 360 degrees and Naviflex nickel-titanium rotary instruments in simulated root canals. In all, 80 canals consisting of four different shapes in terms of angle and position of curvature were prepared by Mity Roto 360 degrees and Naviflex instruments using the techniques recommended by the manufacturers. This study describes the efficacy of the instruments in terms of preparation time, instrument failure, canal blockages, change in canal length, and three-dimensional canal form. Overall, the mean preparation time for canals prepared using Mity instruments was 5.99 min and 5.81 min when using Naviflex instruments. Canal shape had no significant effect on the speed of preparation with either instrument. No instruments separated during the study; however, 14 Naviflex and 2 Mity instruments were deformed. Canal type did not influence significantly the tendency of either instrument to deform. None of the canals became blocked with debris during preparation. The majority of canals prepared by both instruments retained their original working length, and there was no significant difference between the canal shapes in terms of the mean loss of distance or category of distance change for either instrument. Apical stops as judged from intracanal impressions were present in 29 (72%) of the canals prepared with Mity instruments and in 33 (82%) of those prepared with Naviflex instruments. However, the majority were judged to be of poor quality. Significant differences (p < 0.05) were noted in the quality of apical stops between the canal types using Mity instruments. Canals prepared with Mity and Naviflex instruments were found to be smooth in the apical half of the canal in approximately one-half of the specimens and coronally in nearly all canals. Neither instrument produced horizontal or longitudinal grooves. Favorable flow characteristics were apparent in over one-half of the canals prepared with Mity Roto instruments; however, nearly all specimens had poor taper. Flow and taper were generally poor in the specimens prepared with Naviflex instruments. Under the conditions of this study, Mity Roto 360 degrees and Naviflex instruments prepared canals rapidly, with no separations, canal blockages, and with minimal change in working length. Although, flow was adequate using Mity Roto 360 degrees instruments, the taper characteristics were less than ideal compromising the three-dimensional form of the canals. Naviflex instruments, while creating better taper, produced poorer flow characteristics. The results suggest that when using Mity Roto 360 degrees or Naviflex instruments, the stepdown sequence should be modified to improve canal flow and taper. Alternatively, an instrument with increased taper should be used to complete preparation before obturation.