This study was conducted to examine the effects of osteoporosis prevention education on Korean breast cancer patients’ knowledge and health promoting behavior. Methods: The participants in this study included breast cancer patients who were registered at a self-help group organized by the Breast Cancer Center of a university hospital in Seoul, Korea. The intervention group received 60 minutes’didactic instruction followed by 30 minutes’ interactive session, and reinforcement education with leaflets was given three weeks after the group intervention. In order to measure the effects of the intervention, this study used valid and reliable scales on the knowledge of osteoporosis and health promoting behavior for preventing osteoporosis. A post-test was conducted 12 weeks after the intervention. Results: The results showed significant improvement in three of the five factors of osteoporosis knowledge including bone physiology, the characteristics of osteoporosis, and preventive behaviors in the intervention group. Overall health promoting behavior for preventing osteoporosis had significantly increased in the intervention group compared to the control group. Conclusion: Osteoporosis preventive education improved breast cancer patients’ knowledge of osteoporosis and health promoting behavior for preventing osteoporosis by inducing their voluntary participation in self-care activities.