Rheumatoid arthritis is an autoimmune and inflammatory disease that can cause chronic pain. The aim of this study was to investigate the effect of acceptance and commitment therapy on muscle pain in patients with rheumatoid arthritis.Material & Methods: This research was a Quasi-experimental design with experimental and control groups. The study population consisted of 245 patients with rheumatoid arthritis under treatment in the physiotherapy ward of Shaheed Beheshti hospital during 2019 in Hamadan. A sample of 30 subject were selected and randomly assigned in two groups of 15 (experimental and control). The pretest was then performed on both groups. The intervention was performed on the experimental group in 8 sessions (one hour) and the control group did not receive any intervention. After the intervention, post-test was performed in two groups. The research tools included Von Korff et al (1992) chronic pain rate questionnaire. Analysis of covariance used to analyze the data. Findings: The results of analysis of covariance show that there is a significant difference between the mean score of the subscale of pain intensity in the experimental and control groups (P <0.05, F = 5.84). The effect of acceptance and commitment treatment on the severity of muscle pain in patients was 0.18. The results of analysis of covariance also show that there is a significant difference between the mean score of the subscale of disability caused by pain in patients in the experimental and control groups (P <0.05, F = 6.52). The effect of acceptance and commitment treatment on disability due to muscle pain in patients was 0.20. Discussion & Conclusion:The results indicate that acceptance and commitment therapy can be used for reduce the severity of pain and disability caused by pain in patients with muscle pain.