This clinical study was conducted to evaluate and compare the post-instrumentation and post-obturation pain with and without occlusal reduction , in two visits root canal treatment, preoperatively and after 6, 12, 24 and 48 hours utilizing a modified visual analogue scale (VAS). Forty-four patients with mandibular posterior teeth with symptomatic irrivirsible pulpitis and symptomatic apical periodontitis without periapical radiolucency underwent two-visit root canal treatment, where root canals were prepared by crown down technique using Reevo-S rotary system and 2.25% NaOCl solution.The patients were divided into two equal groups (n=22). The clinical trial was a double-blinded, where study participants and assessor did not know which intervention was received.Post-operative pain was assessed using the visual analogue scale (VAS) pre-operatively, 6 hours, 12 hours, 24 hours and 48 hours after post-instrumentation and 6 hours, 12 hours post-obturation.All demographic data, clinical findings and VAS scores obtained from patients were statistically analyzed.Results showed that there was no statistically difference between the two groups regarding the demographic data, prevalence of pre-operative pain, after 6 hours, 12 hours, 24 hours and 48hours. - Within the limitations of this study, it could be concluded that:1)) There is no enough evidence on the effectiveness of occlusal reduction on reducing post-operative pain in mandibular posterior teeth with symptomatic irreversible pulpitis and symptomatic apical periodontitis. 2) ) Pain intensity decreased by time in both treatment groups.