The preparing of cervical third is a manoeuvre that tryes to make easier the acess to the root canal. The excess erode of this regions, in special the intern side, can cause accidents prediction unfavourable. Objectives: The objective of this study is to value comparing the erode of cervical third using Gates-Glidden and Largo drills and ProTaper System. Methods: Thirty medial root of inferior molar were included in ortofitalic resin and secctioned transversally, giving slices 2mm each one thickness. The specimen were divided in four groups. Group I – control, with no prepare; Group II – cervical prepare using Gates-Glidden drills #1, 2 and 3; Group III – using Largo drills #1, 2 and 3 and Group IV – ProTaper System (Shapping files – SX, S1 and S2). The slices that mean cervical third from each group were digitalized before preparing and after each drills used. The eroded area was measured using the Imagelab software, then the results were compared. Results: The results showed that Largo drills (Group III) gave the biggest erode, followed by Gates-Glidden (GII). The statistic analysis by Kruskal Wallis showed statistical differences between groups I and II and II and III. Conclusion: We concluded that the ProTaper System prepared on a safe way the cervical third promoting less erode inside the canal.