INTRODUCTION: Apical transportation results in an inadequately clean apical area and makes apical seal more difficult thus affecting the final outcome and may lead to failure. OBJECTIVES: The aim of this study was to compare between the effect of two rotary nickel titanium systems after using final apical strokes on canal transportation and apical straightening of the canal. MATERIALS AND METHODS: Sixty mesiobuccal canals of extracted mandibular first molars with root canal curvature angle (25

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... o 45 degree) were selected. Canals were divided into two groups (n=30). In group I canals were prepared with OneShape file, while in group II, canals were prepared with Revo-S system. Each group was then sub-divided into two sub-groups according to the motion used (pecking or circumferential filing). Sub group Ia (OneShape file) and sub group Ib (Revo-S system) were prepared with pecking motion while sub group II an and sub group II b were prepared with circumferential filing motion. A radiographic platform was used to obtain accurate radiographs. All specimens were radiographed from proximal and mesiodistal views. To measure apical transportation, the pre and post-instrumentation radiographs were exported from the Kodak software into Photoshop program. Then a grading system was used to assess the amount of transportation. Based on apical curvature measured before and after instrumentation (using Image J program), canal straightening was determined as the difference between these two measurements. RESULTS: Regarding apical transportation, no statistical significance was found between both groups and the sub groups. Apical straightening was evaluated, and no statistical significance was found between the groups. However, circumferential filing produced more straightening. CONCLUSIONS: OneShape file and Revo-S system maintained the original canal curvature with no significant differences between the files. Circumferential filing motion with both systems resulted in higher values of straightening when compared to pecking motion.