||Background: The physical and psychological symptoms occurring after diagnosis and during chemotherapy have a negative effect on the quality of life (QOL) of breast cancer patients. Anxiety and depression are also linked with a deprived QOL and prognosis. This study evaluated the effect of peer support intervention on knowledge about chemotherapy, self-efficacy, empathy, consumer satisfaction, anxiety, depression and QOL of female breast cancer patients on chemotherapy in Yangon, Myanmar.Methods: A randomized controlled trial was conducted at Shwe Yaung Hnin Si Cancer Foundation clinic in Yangon. A total of 74 patients participated and they were assigned randomly into an intervention or a control group. The intervention group received peer support intervention including individual counseling, group meeting, telephone support, and education program during chemotherapy. Data collection was done by interviewer-administered questionnaires at baseline, post-intervention and 2 months follow-up. Independent t-test, chi-square test, analysis of covariance (ANCOVA) test, Quade’s test for non-parametric ANCOVA, Mann-Whitney U test, Wilcoxon signed-rank test, and linear mixed models with random intercepts were used in data analysis.Results: At baseline data collection, there was no significant difference between the intervention and control groups in socio-demographic characteristics, medical history, knowledge about chemotherapy, self-efficacy, empathy, anxiety, depression, global health status/QOL, functioning scores and symptoms scores in QOL, except for role functioning (p=0.019). After the intervention, the intervention group had significantly greater mean scores in knowledge about chemotherapy (p<0.001), self-efficacy (p<0.001), empathy (p<0.001), global health status/QOL (p=0.017), physical functioning (p<0.001), role functioning (p<0.001), emotional functioning (p<0.001), cognitive functioning (p=0.002), social functioning (p=0.002), body image (p=0.032) and future perspective (p=0.002) than the control group. Moreover, the intervention group had significantly smaller mean scores in anxiety (p=0.013), depression (p<0.001), fatigue (p=0.009), and nausea & vomiting (p=0.022) than the control group. At follow-up data collection, the intervention group had significantly greater rate of increase in emotional functioning (p=0.017) and future perspective (p=0.030) than the control group. The intervention group had significantly greater rate of decrease in anxiety (p=0.009), depression (p=0.002) and breast symptoms (p=0.014) than the control group. Besides, the intervention group had significantly lower insomnia score (p=0.016) than the control group.Conclusion: The peer support intervention was effective on improving the knowledge about chemotherapy, self-efficacy and empathy status, and lessening the anxiety and depression status of the participants immediately after the intervention. Regarding the QOL, the intervention program was effective to improve global health status/QOL, physical functioning, role functioning, emotional functioning, cognitive functioning, social functioning, body image and future perspective of the participants. It was also effective to diminish the fatigue, and nausea and vomiting symptoms of the participants immediately after the intervention. The intervention program was also effective on improving emotional functioning and future perspective, and diminishing anxiety, depression, insomnia and breast symptoms of the participants at two months after the intervention. Therefore, the model of the intervention program of this study should be implemented among the breast cancer patients in the future.