Purpose: This study was to determine the effects of abdominal breathing training using biofeedback on stress, immune response, and quality of life. Method: The study design was a nonequivalent control group pretest-posttest, quasi-experimental design. Twenty-five breast cancer patients who had completed adjuvant chemotherapy were enrolled. The experimental group(n=12) was provided with abdominal breathing training using biofeedback once a week for 4 weeks. State anxiety, cancer physical symptoms, serum cortisol, T cell subsets(T3, T4, T8), NK cell and quality of life were measured both before and after the intervention. Result: Though state anxiety, cancer physical symptoms, and serum cortisol were reduced after 4 weeks of abdominal breathing training using biofeedback, there was no statistical significance. It showed, however, improvement in quality of life (p=.02), and T3(p=.04). Conclusion: Abdominal breathing training using biofeedback improves quality of life in breast cancer patients after a mastectomy. However, the mechanism of this beneficial effect and stress response requires further investigation with special consideration in subject selection and frequency of measurement. Nurses should consider this strategy as a standard nursing intervention for people living with cancer.