Purpose: This study was conducted to assess and compare the effect of XP-endo Finisher on post-instrumentation pain after using either One Shape single-file rotary system or ProTaper Universal multi-file rotary system in root canal preparation. Materials and Methods: Forty-eight consented patients, with 52 single-rooted teeth requiring endodontic treatment were selected for this study based on inclusion/exclusion criteria. Teeth were randomly assigned into two equal groups according to the NiTi rotary system used for root canal preparation, Group (I), One Shape rotary system was used and Group (II), ProTaper Universal rotary system was used. Each group was further subdivided into two equal subgroups (A and B) according to whether XPendo Finisher was used in finishing the root canal preparation or not. Pain Intensity was measured using modified visual analogue scale at 6, 12, 24 and 72 hours after root canal instrumentation. Results: The patients' highest mean pain scores occurred in the first 24 hours and significantly declined at the subsequent observation time of 72 hours. ProTaper Universal groups showed the highest mean pain scores, while the lowest mean pain scores were found in One Shape groups with no significant difference. The highest mean pain scores were found in XP-endo Finisher subgroups, while the lowest were found in No XP-endo Finisher subgroups with no significant difference. Conclusion: One Shape system produced less postoperative pain when used in conjunction with XP-endo Finisher.