The aim of the present study was to compare post-operative pain after the use of 1.3% NaOCl and 5.25% NaOCl irrigant concentration in a parallel, randomized, controlled clinical trial using VAS preoperatively, immediately after cleaning and shaping, after 3 hours, after 24 hours, after 48 hours, after 7 days and post-obturation. Fortyfour patients with non-vital mandibular molar underwent two-visit root canal therapy, where root canals were prepared using NiTi ProTaper rotary system. Results showed that high NaOCl concentration seems to cause more early post-operative pain than low NaOCl concentration in cases with symptomatic non-vital pulp