Background and objective: Pain is one of the most important postoperative complaints of patients, which can be caused by the inflammation of the surgical site. Chemical and herbal (e. g. evening primrose) analgesics can be used to mitigate the pain. Evening primrose has such properties as anti-inflammatory impact, amendment of deficient essential omega-6 fatty acids, improved synthesis of vasodilator eicosanoids, enhancement of blood flow in the nerves, etc., which can also be useful in the treatment of inflammation-induced pain. This study was conducted to investigate the impact of evening primrose on the reduction of post-appendectomy inflammation and pain. Materials and methods: This study was a double-blind clinical trial performed on 80 patients with pain at the surgical site after appendectomy at the Shahid Beheshti Emdad Hospital in Sabzevar. Patients were randomly divided into two groups of 40 individuals. Following full recovery and onset of pain, each of the intervention and control groups received one evening primrose and placebo capsules, respectively, 3 times every 30 min. The symptoms were then assessed using a visual pain scale and the McGill pain questionnaire. Results: The results of this study showed that the use of evening primrose capsule could reduce the examined postoperative pain (p < 0. 000). Conclusion: Considering the anti-inflammatory and analgesic effects of evening primrose capsule, the use of this herbal drug is recommended for the treatment of postoperative pain instead of using non-steroidal analgesics with digestive complications. Keywords: Evening primrose, pain, placebo.