Objective: The aim of this study was to evaluate the effect of ultrasonic, sonic and conventional irrigation methods on postoperative pain in acute irreversible pulpitis cases after single-visit endodontic treatment. Material and Methods: A total of 60 patients with acute irreversible pulpitis in mandibular first molars with no periapical pathology were selected and divided into three groups on the basis of irrigation method used, with 20 patients in each group (n=20). In Group A canals were irrigated with side vent endodontic needle (NI). In Group B ultrasonic activation was performed using ultrasonic tips(PUI) and in Group C sonic activation was performed by using Endoactivator(EA)(DENTSPLY).Patient pain intensity after the root canal treatment was recorded at 24,48, and 72 hours using Visual Analogue Scale (VAS). Results: Data was analysed using SPSS version 21. Means of the three groups were compared after 24 hours, 48 hours and 72 hours using one way ANOVA. In the present study, the mean scores of PP on VAS scale in conventional irrigation group were significantly higher than other 2 groups at 24 hr, 48 hr and 72hr although no significant difference was observed between ultrasonic and endoactivator groups. The least pain scores were recorded in endoactivator group. Conclusion: After root canal treatment in lower molar teeth with symptomatic irreversible pulpitis, the highest post operative pain was recorded in conventional needle irrigation group in the first 24 hrs.