It is generally known that there are not enough opportunities for Thai youths to have sex education. Most of them have experienced sexual learning thorough obscene media, which then produces undesirable sexual knowledge among them. As a consequence, they confront problems related to sexual intercourse without the knowledge of sexual education. These problems include unwanted pregnancy, unsafe abortion and other reproductive health problems. This research, therefore, aimed to examine learning patterns of sex education among Thai youths and to examine the relationship on how sex education influenced sexual behaviors of youths. The research employed secondary data from the “Project on the Prevention of HIV Infection in Adolescents through Life Skill Training and Community Counseling Service” conducted by the Institute for Population and Social Research, Mahidol University, between August 13 and September 15, 2006. In the Project, 411 teens and young adults aged between 12 and 24 were employed as the sample group. These sample groups were from schools, colleges and universities in Bangkok and around its outskirts, Lampang, Khonkean and Nakhon Sri Thammarat. Findings from this research reveal that most of the male youths obtain sex education through obscene media, such as watching pornographic movies, more than any other activity. Female youths obtain their sex education by personal experience, spending time with a partner or a close friend. The research found that the majority of Thai youths still think that parents are the first people who should provide sex education for their children. Such education should involve various aspects of sex such as knowledge of sexually transmitted diseases, which is unknown among many teens. Although the majority of youths prefer to be sexually educated by their parents, those who have already had sexual intercourse with a partner prefer to receive advice from their friends, rather than their parents or teachers, if they encounter any sexual problems. The researchers recommended that the sex education curriculum should be developed appropriately according to age. This should involve youth campaigns on sexual behaviors in various educational institutions. Such campaigns would open floors for youths and parents to be campaign organizers who would also receive support and guidance from teacher-mentors. Finally, this activity would offer appropriate sexual knowledge and correct understanding of sex within families as well as in educational institutions.