Introduction: Chest pain is an unpleasant and the most common symptom in patients suffering from unstable angina. This study was implemented to investigate the effect of Thai massage on severity of pain in patients with unstable angina. Methods: This study was a randomized controlled clinical trial that carried out on 70 patients with unstable angina who had been hospitalized in hospital affiliated to Ahvaz Jundishapur University of Medical sciences, Ahvaz, Iran. The participants were allocated randomly to either the intervention (n=35) or control group (n=35) between August 1, 2016 and April 30, 2017. The patients in the intervention group received routine care beside and Thai massage with the duration of 30 minutes in two consecutive days. The patients in the control group received routine care. The intensity of pain was assessed by using the numeric rating scale, five minutes before and immediately, 15, 30 and 60 minutes after the intervention. The independent t-test and repeated measures analysis of variance (ANOVA) were used to analysis data through SPSS 13. Results: After the intervention, the severity of pain was significantly diminished by about four measurement points (P=0.001) with the linear pattern of pain reduction. Nonetheless, in the control group, no statistically significant difference in the intensity of pain was found. Conclusion: This study showed that Thai massage as non-invasive approach, concomitant with standard nursing care, has the potential to decrease chest pain. Nurses should be taught about pain relief methods for improving the quality of patient care.