Abstract : Usualy, ultra sonic is used for cleaning of files for root canl treatment. And then, files are sterilizing with gas. The changing properties of files will occurred by cleaning and sterilizing procedure. In this experiment, files were observed by SEM after ultra sonic cleaning, in several conditions. In the results, blades of stainless steel files became flat in the case of contacted with stainless steel files or nickel-titanium files intentionally, however, any changes were not observed on nickel-titanium files.