PURPOSE: This study examined diabetes knowledge, self-efficacy, and health promoting behaviors, and investigated factors affecting health-promoting behaviors of middle-aged people. METHODS: From August to September 2014, a convenience sample of 264 subjects was recruited from B city and G province. The data analysis was done by chi2 test, t-test, Pearson's correlation coefficient, and stepwise multiple regression. RESULTS: Diabetes knowledge was 11.3+/-3.63 in men and 12.7+/-3.74 in women. The average levels of self-efficacy and health promoting behaviors of subjects were similar with other middle-aged individuals. Diabetes knowledge, self-efficacy, and health promoting behaviors had a positive correlation with each other in both men and women. To test the mediating effect of self-efficacy in the relationship between diabetes knowledge and health promoting behaviors, a Sobel test was performed and Z-scores of 3.698(p<.001) in men and 2.748(p=.006) in women were obtained. CONCLUSION: Diabetes prevention education is recommended for the middle-aged community. When developing diabetes prevention program for middle-aged people, such programs should consider self-efficacy, especially in women.