INTRODUCTION: The preparation of the root canal space includes debridement, shaping and apical preparation. These procedures are challenging and constitute. The recent introduction of automated techniques for canal preparation has created considerable interest. The aim of this study was to test shaping ability of one hand instrumentation (passive step-back technique) and new rotary NiTi system (Mtwo instrumentation) in curved root canals. MATERIALS AND METHODS: A total number of 40 curved molars selected for this in vitro study, 20 canals for each group. The mean of canal curvature in studied groups was similar. After preparation of access cavity, samples were divided into two groups. A custom made block was used for standardization of radiograph beam angel before and after preparation. In group 1, samples instrumented with passive step-back technique using stainless steel K-file, Gates Glidden and Pesso reamers. Mtwo NiTi rotary file and Endo IT power driven motor were used for instrumentation of samples in group 2. Transportation of canals curvature (loss of primary curvature) after canal preparation was assessed for each sample with the aid of AutoCAD 2007 software. Stereomicroscope provided data for measurement of changes in working length after canal preparation. RESULTS: The mean of changes in canals curvature (canal transportation) and mean of working length changes in group 1 (passive step-back technique) were 11.77 degree and 0.202 mm (P<0.001) and in group 2 (rotary Mtwo) were 5.58 degree and 0.202 mm (P<0.001). According to the present results, the original canal curvature was maintained significantly better with automated Mtwo files than with hand instrumentation. CONCLUSION: These can be hypothesized that the instrumentation with Mtwo NiTi rotary file preserves the canal curvature adequately.