The purposes of this study are 1) to describe the health risk behaviors of Thai youth concerning alcohol drinking, smoking, and exercise, 2) to examine the correlation between the personal, familial, psychological and supportive factors and the health risk behaviors of Thai youth, and 3) to investigate the predictive factors that influence the health risk behaviors of Thai youth. The study used secondary data from the Survey of the Youth’s Opinions and Needs on the targets of Children and Youth Development in the 8th National Economic and Social Development Plan by the Institute of Population and Social Research, Mahidol University in 1998. The sample includes 2,306 youths, aged from 15-24 years. Frequency, percentage, mean, standard deviation, Pearson’s Product Moment Correlation Coefficient and Stepwise Multiple Regression Analysis were used for analysis. Overall, most of the youth have low-risk behaviors (76.5%), followed by non-risk behaviors (14.5 %) and high–risk behaviors (9.0 %). The majority of youth have non-risk behavior in alcohol drinking (never drink 48.3%),smoking (never smoke 75.1%) and low-risk behavior in exercise (irregularly exercise (50.5%)).Factors significantly related to health risk behaviors are sex, age,education of youth, education of father, education of mother, family size, self-esteem, exposure of information and health service utilization. Only age, sex, education of youth, self-esteem and health service utilization affect health risk behavior and can predict the risk behavior significantly (24.8 percent). From the results of the study it is suggested that self-esteem and selfconfidence should be cultivated among Thai youth to create motivation, values of life and proper health. Health promotion school projects and a youth network should be in every school to encourage proper health behaviors. Advertising and sales promotions that induce or stimulate youth to have risk behaviors should be prohibited, together with a measure for increasing tax on alcohol and cigarettes. The government, concerned organizations, the community, parents, teachers should pay more serious attention and continuously watch risk behaviors of youth.