Within the last 15 years, the endodontic community has experienced a shift from the use of conventional stainless steel instruments to the use of Ni-Ti instruments. Stainless instruments have shown to be very rigid and can cause ledging and perforation during treatment. On the other hand the incidence of ledging and perforation is greatly reduced when using Ni-Ti instruments. Once Ni-Ti instruments were introduced and combine with rotary instrumentation, it produces faster, rounder and more centered preparations. A lot of research has been done along the years to evaluate rotary Ni-Ti instruments yet few studies have been done to assess the surface properties of these instruments. Assessment of these instruments may shed a light on the issue of number of instruments uses and may allow the discovery of "early warning signs" that could help clinicians in judging when to discard them.This study was designed to compare both(Revo-S and HeroShaper) Ni-Ti rotary files in shaping, centering ability andappearance of root canals.1.Selection of the samples:A total of (84) permanent human mandibular first molar tooth was used in this study. All samples were decoronated at the cemento-enamel junction. Prior to cleaning and shaping of the root portions, a six sided plastic mold (20 x 20 x 16 mm) with 25 holes was designed and constructed to create a 25 cylindrical acrylic blocks containing the root portionsGrouping of samples: The samples were randomly divided into 2 main groups 42 teeth each, according to instruments system that was used during root canal preparation.Group I: 42 teeth instrumented according to manufacture instruction using HeroShaperwhich further subdivided into 2 subgroups according the irrigation regimes 21 teeth each.Group II: 42 teeth instrumented according to manufacture instruction using Revo-S which further subdivided into 2 subgroups according the irrigation regimes 21 teeth each. Part 1: shaping abilityTwenty eight mesiobuccal root canals of extracted permanent mandibular first molars. Samples were divided into two group according to instruments used of canal preparation where, group a: prepared with Revo-S and group B: prepared with HeroShaper rotary instruments. Group A: 14 samples prepared by Revo-S file for evaluation of the angel of curvature and position of canal curvature by cone beam computed tomography.Group B: 14 samples prepared by HeroShaper file for evaluation of the angel of curvature and position of canal curvature by cone beam computed tomography.All samples were photoradiographed by cone beam computed tomography for pre-instruments measurement of angel of the curvature according to schneider’s technique.After instrumentation, each sample in group A and B were cone beam computer tomography to determine the post-instrumentation angel of the curvature. Part2: centric ability:Twenty eight mesiobuccal root canals of extracted permanent mandibular first molars. Samples were divided into two group according to instruments used of canal preparation where, group A prepared with Revo-S and group B prepared with HeroShaper rotary instruments. Group A: 14 samples prepared by Revo-S file for evaluation of the centering ability in the term of dentin thickness by cone beam computed tomography.Group B: 14 samples prepared by HeroShaper file for evaluation of the centering ability in the term of dentin thickness by cone beam computed tomography.All samples were photo radiographed by cone beam computed tomography for pre-instruments measurement of the dentin thickness at three fixed points along the canal. After instrumentation, each sample in group A and B was photo radiographed by cone beam computer tomography to determine the post-instrumentation of the dentin thickness. Part3: appearance of root canal Twenty eight mesiobuccal root canals of extracted permanent mandibular first molars. Samples were divided into two groups according to instruments used of canal preparation; then the two groups were subdivided into subgroups according to irrigtion used where, group A prepared with Revo-S and group B prepared with HeroShaper rotary instruments. Group A: 14 samples prepared by Revo-S file and difference irrigation (NaOCl and NaOCl+EDTA) for evaluation of the root canal by scanning electron microscope.Group B: 14 samples prepared by HeroShaper file and difference irrigation (NaOCl and NaOCl+EDTA) for evaluation of the root canal by scanning electron microscope.The teeth were splitted with a surgical chisel and mallet into two halves; visual inspections of all sectioned halves to detect only intact halves which were used for microscopic evaluation. The specimens were examined from coronal to apical.The results of shaping ability:a.Canal curvature: In (buccal view) there was no statistically significant difference between the two systems. In (mesial view) there was statistically significant difference between the two systems.b.Canal transportation:In (mesio-distal direction) after using Revo-S, there was no statistically significant difference between the two systems at three levels. After using Hero Shaper, there was no statistically significant difference between the two systems at three levels. In (bucco-lingual direction) after using Revo-S, there was no statistically significant difference between the two systems at three levels. After using Hero Shaper, there was no statistically significant difference between the two systems at three levels.The results of centric ability:In (mesio-distel direction) after using Revo-S, there was no statistically significant difference between the two systems at three levels. After using Hero Shaper, there was no statistically significant difference between the two systems at three levels. In (bucco-lingual direction) after using Revo-S, there was no statistically significant difference between the two systems at three levels. After using Hero Shaper, there was no statistically significant difference between the two systems at three levels.The results of appearance of root canal:•Using EDTA + NaOClat the coronal segment, there was no statistically significant difference between the two systems.At the middle segment, there was a statistically significant difference between the two systems. Revo-S showed statistically significantly higher mean debris score than Hero Shaper. At the apical segment, there was no statistically significant difference between the two systems.•Using NaOClat the coronal segment, there was no statistically significant difference between the two systems. At the middle segment, there was no statistically significant difference between the two systems. At the apical segment, there was no statistically significant difference between the two systems.Conclusion: •Combination of NaOCL and EDTA were found to be mandatory in cleaning efficacy of root canal wall.•No system was able to clean the canal from debris.•Cone beam computed tomography was found to be useful in research especially with the proper software.