Aim of the study: was to compare the shaping ability of One Shape, Revo-S NiTi rotary instruments, and stainless steel K-Flexofile instruments when used to prepare curved root canals.   Methodology: A total of sixty mesiobuccal root canals of extracted human permanent mandibular molars were used. Those had curvature ranging between 200- 400, radius of curvature between 4-10 mm, and length of curved part between 6-8 mm. The samples were divided into three groups of 20 teeth each according to the instrument used. Using Digora system, the degree of straightening was calculated by comparing pre and post-instrumentation radiographs. Using cone beam computed tomography, mesial and distal dentin thicknesses were measured pre and post- instrumentation, and then applied in an equation given by Gambill et al., to calculate the centering ratio and transportation of the canals. Both rotary systems showed good centering ability and less canal straightening and transportation compared with stainless steel K-Flexofile, with no significant difference between the rotary systems.   Results: Tukey's pair-wise test showed that K-Flexofile showed the highest mean degree of straightening, degree and direction of canal transportation and the centering ability while One Shape had the lowest mean. No statistically significant difference was found between One Shape and Revo-S. Conclusion: Both rotary systems caused less transportation and remained better cantered in the canal than stainless steel K-Flexofile instruments, with no significant difference between One Shape and Revo-S.