Statement of Problem: One of the difficulties that a dentist encounter during root canal treatment is the cleaning and shaping of curved root canals. While doing this procedure the probability of zipping, perforation, apical transportation and canal curvature change increases that all of those are the result of not following the original root canal path by the file. This will fail the prognosis of root canal treatment.Purpose: This in vitro study was undertaken to evaluate the effects of a hand and a rotary instrumentation technique (passive step back and profile system) on canal curvature of mesial roots of human extracted molar teeth.Materials and Methods: In this experimental study, 40 mesial roots of first molar teeth with (20-40 degree) curvature were used. The roots were divided randomly into two groups of 20 and the canals were prepared by a hand instrumentation technique (passive step-back) and a rotary (profile system). Pre and post instrumentation radiographs were taken by radiographic platform. The amounts of alteration in canal curvature were measured by the aid of Adobe Photoshop 7 and Auto CAD 2000 software. Results were analyzed using Man-Whitney test.Results: After instrumentation, canal curvature showed 85% reduction in passive-step back and 65% in profile system experimental groups. Mean canal curvature reduction in passive-step back was 3.15+1.94 while in profile system was 1.85+1.67. There was statistically significant difference between two experimental groups (p<0.05).Conclusion: Under the condition of this study, it seems that better maintenance of the original canal curvature in profile systems was due to the difference, between the properties of Niti and Stainless steel files. Key words: Passive step-back, Profile system, Canal curvature, First molar teeth