Objectives: The aim of the study was to compare postoperative pain on using 1.3% and 5.25% sodium hypochlorite irrigating solution in two-visit root canal treatment at different time intervals: preoperative, immediately post-instrumentation, after 3 hours, 24 hours, 48 hours and 7 days post-instrumentation and post-obturation utilizing a modified visual analogue scale (VAS). Materials and methods: Forty-four patients with non-vital, mandibular first and second molars teeth with or without periapical radiolucency were selected. Medical and dental histories and chief complaints were obtained. Clinical and radiographic information was recorded. After access and working length determination, the root canals were instrumented in a crown-down technique using NiTi ProTaper rotary system and a 27- gauge, side vented needle for irrigation which penetrated to 3 mm before the working length. In group I, 1.3% sodium hypochlorite solutions and in group II, 5.25 % sodium hypochlorite was used as an endodontic irrigants. All canals were then dried and the access cavity was closed with cavit temporary restoration. The patient assessed their severity of pain using a modified visual analogue scale pre-operatively and in between the two visits immediately post-instrumentation, 3 hours, 24 hours, 48 hours and 7 days after instrumentation, then post-obturation. All demographic data, clinical and radiographic findings and VAS scores obtained from patients were statistically analyzed. Results: there was no statistically difference in the prevalence of pre-operative pain, pre-operative signs and symptoms, periapical radiolucency, over-instrumentation and over-obturation between the two groups. On the other hand, when comparing mean VAS scores in the two groups, Group B (5.25% NaOCl) showed statistically significantly higher mean VAS scores than Group A (1.3% NaOCl) with the highest difference after cleaning and shaping by 24 hours. Conclusion: sodium hypochlorite at higher concentration (5.25%) caused more post- instrumentation pain than NaOCl at lower concentration (1.3%) in cases with non vital pulp. Pain intensity decreases by time especially after 24 hours post-instrumentation.