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The purpose of this study was to evaluate the shaping ability of S5 rotary system in relation to Pro Taper Universal system in preparation moderately curved root canal their effect on the canal transportation, centering ability and canal curvature change. Materials and Methods: Collected thirty mesiobuccally root canals of extracted human mandibular first molar were divided into two equal groups; Pro Taper and S5. The samples were mounted in two moulds of acrylic resin for pre-instrumentation imaging using cone beam computed tomography (CBCT). After root canal instrumentation, post-instrumentation imaging using CBCT was done with the exacted parameters of the pre-instrumentation imaging. The pre and post-instrumentation CBCT images were superimposed to measure the dentin thickness at three root canal levels; Canal transportation and centering ability were recorded according to formula introduced by Gambill., et al. (1996), while the change in the angle of canal curvature was recorded following Estrela., et al. (2008). Statistical Analysis: Data was tabulated and statistically analyzed; Independent samples t-test was used for comparing the variables between the two groups at each root canal level and also was used for comparing the percentage of change in the canal curvature angle between the two groups. The significance level is considered at P ≤ 0.05. Results: There was no statistically significant difference between the two rotary systems in canal transportation except at the coronal level; S5 recorded statistically lower mean value of root canal transportation than Pro Taper system. The centering ratios and the changes in the angles of canal curvature after the use of both systems were not statistically significant. None of the used systems showed perfect centering ability. Conclusion: Under limitations of this in vitro study, it can be concluded that; preparation of moderately curved mesio-buccal root canals with S5 was associated with lesser coronal transportation and similar middle and apical transportation, centering ability, and canal curvature change when compared to Pro Taper. anatomy specifically root canal curvatures present a challenge to enlarging instruments and might accompany by a number of