Objectives:The purpose of this study was to evaluate by a double periapical radiographic exposition, the morphologic alterations in the apical third of the root canals prepared by the HERO 642 rotary system and NiTi handly files. Methods:40 human mandibular molars,with rizogenesis process concluded, obtained from Banco de Dentes Humanos do Departamento de Protese e Cirurgia Buco-Facial da Universidade Federal de Pernambuco,with the objective of comparing the morphologic alterations, in the apical third, produced by the root canal preparation, in wich the final apical instrument was one with the diameter D0 equivalent at 0,25mm. The radiographic images were analised by three independent examinations, formaly endodontistis, previously adjusted and the results were submited by a statistic analysis, the Fishers exact test was utilized to compare the rate of bypass obtained by two tecniques. Results: The preparation of nickel-titanium handly files showed 25% of root canals with bypass in the apical third while Hero 642 rotary system showed 10%. Conclusion: The data obtained allowed to conclue that Hero 642 rotary system showed less root canals with bypass in the apical third than nickel-titanium handly files, although, these results were not statisticly significant.