Purpose: This study was conducted to compare and evaluate the influence of motion pattern on shaping ability of two single file systems (Reciproc and Neoniti) in severely curved mesiobuccal (MB) root canal of human mandibular first molars using cone beam computed tomography (CBCT) scanning. Materials and Methods: Forty non-calcified Mesio-buccal root canals with mature apices and apical curvature of 20-45o and Radius less than 15mm were selected from extracted human mandibular first molars. The samples were divided according to the instrument (n=20) into group I (Reciproc R 25/08) and group II (Neoniti R 25/08). Each group was subdivided according to the motion (n=10) Subgroup A: reciprocation motion (RM) Subgroup B: full rotation motion (CM). After preparation, the amount of apical transportation, centering ability and radius change were assessed by evaluating pre- and post-instrumentation CBCT scans by superimposing in four section (2, 4, 6 and 8 mm from apical foramen). the significance level was set at P ≤ 0.05. Results: There was no ststistical significant difference in canal transportation among the subgroups at three studied levels (2, 4, and 8 mm from the root apex) (P>0.05) in both Bucco-Lingual and Mesio-Distal directions, but at 6mm level where Neoniti CM peroduced significantly more amount of canal transportation than Neoniti RM in MD direction. The ability of instruments to remain centered in prepared canals at 2 and 4 mm levels was significantly higher in Neoniti RM (P<0.05) in MD dimension. The centering ratio at 6 and 8 mm level and in BL dimension were not significantly different among the tested subgroups(P>0.05). The change of the radius values and percentage produced no statistically significant difference. Conclusion: The motion pattern has an impact on the shaping ability of the severly curved root canal. Reciprocation motion enhances transition of the file inside the canal with minimal procedural errors.It seems that the difference between Neoniti and Reciproc is of little importance concerning apical transportation and centering ability and Radius change. Also both files preserve the root canal anatomy.