Aim: The purpose of this study was to compare the canal centreing ability of three nickel-titanium (NiTi) rotary systems (Mtwo, Twisted Files and Revo-S). Materials and methods: Sixty mesial root canals of mandibular first molars with curvatures ranging from 25 to 35 degrees were randomly divided into three groups. Root canals were instrumented using Mtwo (VDW, Munich, Germany), Twisted Files (TF) (SybronEndo, Orange, CA, USA) or Revo-S (RS) (Micro-Méga, Besançon, France) rotary NiTi instruments. The roots were sectioned horizontally at 3 mm, 6 mm and 9 mm from the root apex. Teeth were reassembled and the canals instrumented according to the manufacturers' guidelines up to master apical file size 30. The cross sections of the root canals were digitally scanned before and after preparation, and image analysis software was used to assess the centreing ability. The three groups were statistically compared using analysis of variance (ANOVA) and the Tukey HSD test (P 0.05). Results: TF produced the lowest total mean centreing ratio (0.042 ± 0.005) at all canal regions (P 0.05). RS produced the highest total mean centreing ratio (0.598 ± 0.12) (P 0.05) followed by Mtwo files (0.194 ± 0.02). Conclusions: Twisted Files provided the least canal transportation, remained centred in the canal and preserved the original canal cross section to a greater degree than Revo-S and Mtwo instruments. Keywords: canal transportation, centreing ability, curved root canals, M-wire