Aim: This study compared the incidence of deviation along curved canals after preparation with three nickel-titanium (NiTi) rotary systems (K3XF, Mtwo and BioRaCe). Materials and methods: Curved canals from 60 resin training blocks were filled with ink and divided into three groups according to the instrumentation system. Pre-instrumentation images were acquired by using a stereomicroscope. All canals were prepared up to an instrument size 35/0.04 at the terminus of the canal. Post-instrumentation images were taken using the same conditions and pictures were superimposed. The amount of resin removed was measured at eight different points, beginning at the apical terminus of the canal. Differences in the mesial and distal aspects were measured to evaluate the occurrence of deviation. Results: Intragroup analysis showed that all instruments promoted some deviation at all levels. There was a significant difference at most of the levels for all groups (P 0.05), except for the 1-mm level of the K3XF (P = 0.099) and Mtwo (P = 0.196) groups, the 3-mm level of BioRaCe group (P = 0.071) and the 7-mm level of the Mtwo (P = 0.051) and BioRaCe (P = 0.257) groups. Deviation was observed at the distal canal wall at levels 3, 4, 5 and 6 from all groups, at level 2 from both K3XF and Mtwo groups, at level 1 from the K3XF group and at level 7 from the Mtwo group. At the mesial wall, deviation was observed at level 0 from all groups, at level 1 for both Mtwo and BioRaCe groups and at level 7 for both K3XF and BioRaCe groups. Mtwo showed significantly lower deviation at level 1 than K3XF and BioRaCe. At level 2, BioRaCe was significantly better than Mtwo and K3XF and Mtwo performed better than K3XF. No significant differences were observed for the other levels. Conclusions: All the rotary NiTi systems showed some deviation during the preparation of curved canals. Mtwo showed a significantly better performance at level 1 and BioRaCe at level 2. Keywords: curved root canals, root canal deviation, root canal preparation, rotary nickel-titanium instruments