The objective of this randomized clinical trial was to evaluate the incidence of postoperative pain after root canal treatment of tooth with irreversible pulpitis of teeth using two different instruments rotary file systems. Material and Method: In this prospective randomized clinical trial, 40 teeth with symptomatic irreversible pulpitis, indicated for root canal therapy, were randomly assigned to two groups according to instrument system used, namely, protaper universal and protaper next. Root canal treatments were performed in single visit. After 12 hours, 24 hours, 72 hours the participants were asked to rate their level of postoperative pain on a visual analogue scale (VAS) (no pain, mild discomfort, moderate pain, and severe pain). Data obtained were analyzed using the chi square test and Statistical significance was defined as p < 0.05. Result: Results showed that there was a significant differences between both rotary file groups. Postoperative pain was significantly higher in protaper universal group than protaper next file group. Conclusion: Postoperative pain was lower in the protaper next file system than in the protaper universal file system after single-visit root canal therapy at 12, 24, and 72 h.