The aim of the present study was to compare the intensity and incidence of postoperative pain after the use of 1.3% NaOCl and 5.25% NaOCl irrigant concentrations in a parallel, randomized, controlled clinical trial. Forty-four patients with non-vital lower first and second mandibular molars with or without periapical radiolucency underwent two-visit root canal therapy, where root canals were prepared using NiTi ProTaper Universal rotary system. The patients were devided into two equal groups (n = 22). The clinical trial was a triple-blinded where the patients, the operator and the assessor were not aware of the intervention applied. Post-operative pain was assessed using a visual analogue scale (VAS) preoperatively, immediately after cleaning and shaping, after 3 hours, 24 houes, 48 hours, 7 days and post-obturation. All baseline demographic data, preoperative clinical and radiographic findings and VAS scores were obtained from each patients and were statistically analyzed. Results showed that there was no statistically difference between the two groups regarding the demographic data, prevelence of preoperative pain and the prevelence of periapical radiolucency. There was a statistically significant difference between group A (1.3% NaOCl) and group B (5.25% NaOCl) in mean VAS scores after 48 hours with group B showing higher score. No significant difference in the incidence of different pain categories (no pain, mild pain, moderate pain and severe pain) between the two groups was detected at any of the time point. • Within the limitations of this study, it could be concluded that: 1- High NaOCl concentration seems to cause more early post-operative pain than low NaOCl concentration in cases with non-vital pulp. 2- Maximum post-operative pain can be anticipated 48 hours postinstrumentation with compelete recession after 7 days in case of non-vital pulp with no or mild preoperative pain. • Recommendations: It could be recommended to use low (1.3%) NaOCl concentration as irrigating solution during root canal therapy to decrease post-operative pain in mandibular teeth with non-vital pulp.