The purpose of this study was to examine the prevalence and correlated factors of sexual behavior among high school students in Seoul A sample of 233 male and 248 female high school students were analyzed using cross-tabulation and logit regression models. Correlated factors examined include type of school, level of mothers education, perceived living status of family, whether family has two parents or not, and whether students have ever lived away from the family, whether students received reproductive health education at school and whether they have friends with sexual experience, whether students have ever smoking and alcohol drinking. The prevalence of alcohol drinking was 73% among boys and 55% among girls and the prevalence of smoking was 64% of boys and 40% of girls, whereas the prevalence of sexual activity was 27% among boys and 15% among girls. Risk taking was more prevalent among boys than among girls. Multiple risk taking behavior was common for both boys and girls. Students who did not have two parents were more likely to engage in risk taking behavior than those who had two parents. For both boys and girls, the factor that affects their own sexual activity most was having a friend who was sexually active and having an experience of living away from their family also increases the odds. For girls, the factor that affects having experience of alcohol drinking and smoking. Receiving reproductive health education at school had no effect on students sexual behavior. Much higher risk taking behavior with sexual behavior among students in Seoul implies that the overall prevalence of risk taking behavior among high school students was likely to rise as South Korea continues its modernization. In-school and community health education programs need to be modified to be effective in protecting students from risk taking sexual behavior.