Background: Among the most prevalent complications of root canal treatment is postoperative pain. The goal of this study was to compare the severity of postoperative pain after root canal preparation with RaCe rotary system as well as hand K-Flexofile. Materials and methods: With 50 mandibular molars in each group, the sample size was of 100 subjects. The study comprised participants who needed endodontic treatment for asymptomatic irreversible pulpits in their mandibular first or second molars with normal periapical radiography images. Periapical radiographs were processed and preserved using a specialised scanner and software interface before being used for additional examinations with Rinn XCP devices and a digital radiography system. A physician divided the 100 subjects into two groups of 50 subjects each after choosing the subjects. Gender as well as the number of mandibular first and second molars with three and four root canals were matched between the two groups. Results: In both manual and rotary groups, severity of postoperative pain significantly decreased from the beginning to the end at all evaluated time intervals (P<0.001). However, comparison of pain severity between the RaCe rotary and hand K-Flexofile groups did not reveal any significant differences between the two groups (P=0.79). In this context, the mean pain severity scores four hours after treatment were 28.34±5.61 in the RaCe group and 36.57±6.21 in the K-Flexofile group. After eight hours, the pain severity scores were 22.32±4.56 and 29.45±4.72 in the rotary and hand file groups, respectively. The pain severity at both intervals in the rotary group was less than the hand file group, but the difference was not statistically significant (P>0.05). At twelve hours, twenty-four hours, forty-eight hours and seven days intervals, the variations in pain severity among the 2 groups were not noteworthy (P>0.05) Conclusion: There were found no numerically considerable variations in pain severity among the two groups at any intervals. Key Words: Manual Instrumentation, Postoperative Pain, Root Canal Preparation, Rotary Instrumentation.