Purpose: Self-efficacy encompass one’s belief in one’s ability to organize and achieve goals. Previous studies have not adequately examined the mediating role of self-efficacy between social support and health promotion behavior. Therefore, this study explored the mediating role of self-efficacy in the relationship between social support and health promotion behavior among older women living alone. Methods: Participants were 145 older women living alone attending a local welfare center for seniors. They completed the Self-efficacy Scale, Medical Outcome Study Social Support Survey Scale, and Health Promoting Lifestyle Profile II. Data were analyzed using Descriptive statistics, Pearson correlation coefficients, Baron and Kenny’s regression analysis and the Sobel test with the SPSS program. Results: The average social support, health promotion behavior, and self-efficacy were not high. Self-efficacy was a partial mediating role in the relationship between social support and health promotion behavior. Social support was positively correlated with self-efficacy (r=.31, p<.001) and with health promotion behavior (r=.43, p<.001), and self-efficacy was positively related with health promotion behavior (r=.39, p<.001). Conclusion: To enhance health promotion behavior in older women who live alone, intervention strategies to increase social support and self-efficacy for these women should be developed.