Background and Aim: Cleaning and shaping is one of the important steps in endodontic treatment, which has an important role in root canal treatment outcome. This study evaluated the rate of file fracture and file deformation in Neolix rotary system and K-files in shaping of the mesiobuccal canal of maxillary first molars with moderate curvature. Materials and Methods:In this ex vivo experimental study, the mesiobuccal root curvature of maxillary first molars was measured by the Schneider's method, and canal preparation was performed in 2 groups of 30 with Neolix rotary system and manual K-files. To determine the fracture rate of files, a file was used until it broke or deformed, and the number of canals cleaned by that file was recorded. The data were analyzed by the Mann-Whitney U test. Results: File fracture rate in the rotary group was slightly higher than that in the manual K-file group but, the frequency of file deformation in manual K-files was slightly more than that in the rotary group. There was no statistically significant relationship between file type and frequency of file fracture or deformation (P>0.05). Conclusion: Manual stainless steel K files and Neolix NiTi rotary files were the same in terms of file fracture and file deformation in preparation of canals with moderate curvature.