Introduction: The purpose of this prospective, randomized clinical study was to compare the results of using low concentration of sodium hypochlorite irrigating solution (1.3% NaOCl) with using high concentration of sodium hypochlorite irrigating solution (5.25% NaOCl) in decreasing postoperative pain.Methods: Forty four patients were recruited with a posterior tooth requiring primary endodontic treatment of non vital pulp. Standard nonsurgical endodontic therapy was performed on both groups in a 2-visit approach. Teeth were randomly treated with 1.3% or 5.25% NaOCl irrigating solution in the first visit till complete instrumentation. Pain levels were assessed by a visual analog scale questionnaire preoperatively, immediately post-instrumentation and after 3 hours, 24 hours, 48 hours and 7 days, then post-obturation.Results: The mean VAS score was more for group A (5.25% NaoCl) immediately post-instrumentation, after 3 hours, 24 hours and 48 hours of instrumentation compared to group B (1.3% NaOCl), it decreased gradually from 24 hour to 7 day post-instrumentation. 1.3% NaOCl group showed statistically significantly higher post-instrumentation pain than 5.25% NaOCl group (P > 0.05).Conclusion: Within the limitations of this study, NaOCl at lower concentration (1.3%) caused less postoperative pain than higher concentration (5.25%).