Objective: The purpose of this study was to investigate impact of knowledge familiarity, and prejudice about mental illness as well as demographic factors on the social distance from mentally ill people, which is a proxy measure of discrimination. Method: To assess the impact of knowledge and familiarity, prejudice about mental illness and demographic factors on the social distance from mental illness, we conducted a telephone survey in South Korea with the responders being nationally representative people who were 18 years old or over (n=1040). Independent samples T-tests, one way ANOVA and linear regression analysis were performed to analyze the results of the survey. Result: The social distance from mental illness decreased as the knowledge and familiarity increased, but the social distance was increased as prejudice was increased. Prejudice had a greater impact on social distance than familiarity and knowledge. Females showed greater social distance than did males. A higher education level had a negative effect on social distance. Conclusion: to reduce the social distance from mentally ill people, efforts to increase the familiarity about mental illness as well as efforts to educate people about mental illness are important.