Background and objective: The background of this study revolves around endodontic treatment, which is a therapeutic approach involving the chemical and mechanical disinfection of root canals, followed by the establishment of a secure three-dimensional seal. The primary objective of this research is to investigate and understand the underlying factors that contribute to flare-ups experienced by patients undergoing endodontic treatment. Method: A total of fifty-two patients were considered participants and were included for history taking, clinical examination, radiographic assessment, and data analysis after receiving Root Canal Treatment (RCT) for different types of teeth (incisors, canine, and premolar) by different students (operators) and different supervising seniors (trainers). During this time, the operator performed RCT on various patients, and the participants were followed up after they completed root canal obturation. Statistical analysis was done by the SPSS program, chi-square and Fisher's exact test were used to determine the correlation between variables. Result: The level of significance used in this research is ≤ (0.05), and the result is significant when (p≤0.05). The main findings were that the frequency and distribution of post-instrumentation pain was (30.8%). According to our data, we found out the highest incidence of flare-up in HMU was related to pulp remnant and periapical pathosis. Conclusion: There are several factors that are related to the prevalence of post-obturation flare-up such as over-instrumentation, pulp remnant, periapical condition, presence of patient medical history, and perforation. According to these factors the dentist should be aware of the possibility of flare-up occurrence, however, it must be borne in mind that theories about these factors cannot be completely conclusive. Keywords: Endodontic Treatment, Flare-up, Postoperative pain.