Introduction: This study compared the effect of 2 rotary nickel-titanium (NiTi) systems; WaveOne and ProTaper Universal on transportation, centering ability and volumetric changes of curved root canals. Methods: Thirty-four mesiobuccal root canals of mandibular molars (with angles of curvature ranging from 25º - 35º) were randomly divided into two groups (n=17); WO and PU groups. Canals were scanned using a Cranex® 3Dx machine (Soredex, Tuusula, Finland) before and after preparation. Root canal transportation and centering ratio were evaluated at 3, 5, 8 mm from the apex. Amount of removed dentin was evaluated as well. The significance level was set at P ≤ 0.05. Results: There was no significant difference between both systems in canal transportation, centering ability and volumetric changes.Conclusions: Both systems can effectively shape curved root canals, while preserving original canal curvature, and without excessive removal of dentin.