Objective: assessment the canal transportation and centering ability of two single file systems (Reciproc and Neoniti) in  curved mesiobuccal (MB) root canal of natural lower first molars, by cone beam computed tomography (CBCT) scanning. Methods: Forty non-calcified Mesio-buccal root canals with complete formed root and apical curvature of 20-45o and Radius ≤ 15mm were chosen from extracted human lower first molars. The samples were divided based on the instrument (n=20) into two main groups group I (Reciproc R 25/08) and group II (Neoniti R 25/08). Each group has another subdivided based on the motion into (n=10) Subgroup A: reciprocation motion Subgroup B: full rotation motion. The apical transportation, centering ability and radius change were measured by pre- and post-instrumentation CBCT scans by superimposing in four section (2, 4, 6 and 8 mm from apical foramen). Values were recorded, tabulated for each group and statistically analyzed. Results: The ability of instruments to remain centered in prepared canals at 2and 4 mm levels was higher in Neoniti reciprocation in MD dimension. The centering ratio at 6 and 8 mm level and in BL dimension were not significantly different between the tested subgroups. The change of the radius values and percentage produced no significant difference. Conclusion: Both Neoniti and Reciproc systems haven't significant difference in canal transportation, centering ability and change in curvature radius expect when using reciprocation motion Neoniti produced significantly better results than Reciproc especially in the apical third.