We investigated the ecological variables of adolescent sexual behavior. We grouped the behaviors into organism, microsystem, mesosystem, and exosystem. The variables belonging to the organism group were sexual attitude, sex, grade, sexual knowledge, sexual education, dating experience, problem behaviors, and self-control. The microsystem variables included the parent-adolescent communication about sexuality, parental monitoring, parent educational background, friend relationships, and school environment. Moreover, the mesosystem variables included family-peer and family-school relationships. The exosystem variables were comprised of neighborhood environment and pornography. The study group included 369 adolescents from the first and second grade of several high school in Daegu, Korea. We found that the factors affecting adolescent sexual behavior included grade, sex, dating experience, self-control, smoking, and pursuing sexual pleasure. As well, contact with deviant friends, father-adolescent communication about sexuality, school type, and attachment to teacher. In addition the family-peer relationship also affected adolescent's sexual behaviors as well as pornography and neighborhood environment. Lastly the variable belonging to the organism group was found to have the greatest effect on adolescent, sexual behavior compared to the other variables.