This study was conducted to examine factors influencing mental health of middle-aged adults(N=203) in relation to sociodemographic variables, health behaviors, and social support. The survey with questionnaire was carried out for the subjects of the residents in two communities of a local city located in Gangwon-do, Korea from September 3 to 15, 2003. Data analysis procedure included stepwise multiple regression using mental health as the dependent variable, and sociodemographic variables, health behaviors, and social support as independent variables. There were significant differences in the mental health by birth place(t=-1.67, p<0.10), religion (t=2.27, p<0.05) and subjective economic status(F=2.29, p<0.10). Mental health showed significant positive correlations with both health behavior (r=0.462, p<0.001) and social support (r=0.142, p<0.05). Stepwise multiple regression analysis for mental health revealed that the most powerful predictor was health behavior(β=0.453, p<0.001). Health behavior and religion explained 24％ of the variance. The results suggest the necessity of a intervention that considers the health behaviors should be included in middle-aged adults so as to promote mental health.