Purpose: This study examined the effects of aerobic exercise using a flex band on the improvement of physical functions & body image in breast cancer women undergoing radiation therapy after a mastectomy. Method: Women with breast cancer(n =26) were assigned to an experimental group(EG, n = 15) and control group(CG, n = 11). The E.G participated in an aerobic exercise program with a 60% to 80% intensity of maximal heart rate for 25 minutes during the main exercise, 3 times per week, for 6 weeks. The EG did not exercise regularly for 3 months before participating in this program. The CG received no exercise treatment during the research period. Data were analyzed using the $x^2-test$ and Mann-Whitney U test by the SPSS version 11.0 program at a 5% significant level. Results: Group analysis revealed that the EGwomen had significantly more improved cardio-pulmonary functions, ROM of the affected shoulder joint, and body image compared to the CG, Conclusion: Aerobic exercise using a flex band may be an effective rehabilitative measure for mastectomy women with respect to cardio-pulmonary functions, ROM, & body image. Further studies are recommended to study early rehabilitation programs within 10 days post-operatively.