Objective: Postoperative pain has been thought to vary according to the instrumentation kinematics. The aim of the present trial was to compare the incidence and intensity of postoperative pain after using Reciproc or One Shape nickel-titanium systems for root canal instrumentation. Materials and methods: One hundred and eight patients with symptomatic irreversible pulpitis were assigned to two groups according to the root canal instrumentation technique, Reciproc (VDW, Munich, Germany) or One Shape (Micro Méga, Besançon, France). Treatment was completed in a single visit. At the end of the visit, participants were asked to rate the intensity of postoperative pain on the numerical rating scale at 6, 12, 24, 48 and 72 hours. Participants were also given ibuprofen 400 mg to be taken in case of intolerable pain and asked to record the number of analgesic tablets taken at these time points. Mann-Whitney test was used to compare between two groups in non-related samples. Friedman test was used to compare between more than two groups in related samples and Wilcoxon signed-rank test was used to compare between two groups in related samples. Results: A statistically significant difference was found in terms of incidence of postoperative pain at 12 hours (P = 0.034) and 24 hours (P = 0.04) postoperatively, with less incidence of pain in the One Shape group compared to the Reciproc group. No differences were found in terms of the intensity of postoperative pain or the analgesic intake at the assessed time points (P > 0.05). Conclusions: Continuous rotary instrumentation was accompanied by a significantly lower incidence of pain at 12 and 24 hours postoperatively compared to reciprocation. The intensity of postoperative pain and analgesic intake were similar in both groups. Keywords: One Shape, postoperative pain, Reciproc, root canal instruments