This study was designed to find a suitable method for estimating and predicting the average life span of rotary files after simulated clinical use. Materials and methods: We used One Shape rotary file system. A total of 120 mesiobuccal root canals from lower 1st molars were used with medium curvatures in the range of 20-40 degrees. All canals were prepared at torque of 4N/cm² and 350 rpm. Files were subjected to autoclaving after preparing each 3 canals. Files were evaluated for deformation by four methods; magnifying loupe, DOM, SEM and atomic force microscope at the base line, after preparing 3,6,9 and12 canals. Scores were recorded. We used a modified form of the classification of Chekka et al to evaluate the deformation of files. Results: Some of -as received- files showed some sort of deformation evidenced by M.L and DOM and this was doubled when evaluated using SEM or AFM. All files developed more deformation after preparing more canals. The score of deformation ranged between 1-67%. No files exhibited fracture before preparing 9 canals. The score of deformation was in direct relation to the number of prepared canals. The significant differences in the tools of evaluation were not significant until preparing nine canals. Conclusion the results reflect the importance of the loupe in detecting major deformations and the DOM in detecting the minor deformations. It also gives a notation about the importance of regular check up of the files after use and using visual aids specially ones with high magnification powers as the DOM. There were significant difference of the results of the evaluation methods after preparing nine canals with atomic force microscope the first detection method followed by SEM, DOM and finally M L the implication of this finding is M.L. and DOM are satisfactory clinical means of evaluation of the files and stating the time of discarding.