The aim of this study is to compare the shaping ability of two rotary Ni-Ti systems and manual stainless steel K-Flexofile and also SEM evaluation of two rotary NiTi systems after instrumentation cycles.Materials and methods: The mesiobuccal canals of human mandibular molars were preferred to plastic standardized blocks to simulate the clinical situation. root canals with angle of curvatures range between 20º and 40º degrees and radius of curvature between 4-10 mm were used in this study. The length of the curved part of the canal was considered a factor affecting the performance of the instrument inside the canal, as the stress acting on a canal instrument is highest in the curvature zone for that, the selected range of the length of the curvature was between 6-8 mm. Direct digital radiography (Digora system) was used for evaluation the degree of canal straightening by comparing pre and post instrumentation curvature. Although the advantages of high image quality, low radiation dose and precision linear measurements, it has limitation of showing two dimensions only of the root canal. CBCT was used in this study for measurement root dentin thicknesses.Results: Regarding the degree of straightening of canal curvature, results showed that Ni-Ti rotary systems, GTX and Revo-S maintained the original canal curvature better than stainless steel files coinciding with other previous studies . On the other hand GTX rotary nickel-titanium had the lowest degree of straightening rather that Revo-S and K-Flexofile. SEM evaluation of Revo-S showed pre-operative defects as machining marks and pitting are characteristic features of manufacturing processes (grinding & etching) in addition to post-operative defects as metal peel, micro fracture and edge irregularity. Post-operative SEM evaluation of GTX show rolling over on the edge of the instrument, but there is no micro-fractures appeared on the edge of instruments.Conclusion: there is no significant differences were found between GTX and Revo-S systems regarding their shaping of the ability in curved canal.It was concluded that new M-wire Ni-Ti of GTX did not improve the centering ability compared to traditional Ni-Ti of Revo-S but it improved proper surface evaluation after used in seven curved canals without crack propagation, micro fractures, metal peal, tip Irregularity and un-winding. So it was safer to use GTX to prepare 7 curved canals than Revo-S.