The purpose of this article was to get hold of adolescent's needs for sex knowledge and to reveal their understanding of sex. These results may be applied to the sex education and counselling for adolescent. The participants of study were 267 students (females, 144 ; males, 123) who were the first and second grade form 1 middle and 2 high schools. They were asked to describe 3 questions that they wanted to know or to learn about sex. Their questions about sex were total 779(girls, subtotal 456, mean 3.2, boys, subtotal 323, mean, 2.6). These questions categorized to 9 items by content analysis. The items were ‘sexual drive, behavior and tendency(229, 29.4%)’ , ‘anatomy and physiology of reproductive system(140, 18.0%)’, ‘reproduction(131, 16.8%)’, ‘concept of sex(31, 4.0%)’, ‘acquaintance between the other sexes(17, 2.2%)’, and ‘the others(9, 1.2%)’ in order of frequency. The most frequent item that the student want to know or team about sex was ‘sexual drive, behavior and tendency’ except girls of sophomore in middle school. There were several features in participants' needs for sex knowledge as respects of the understanding of sex- biological sex, gender, and sexuality. The prominent feature in knowledge need of bio logical sex was that the participants had the interests according to their biological sex. They had the negative attitude to the phenomenons (ex, menstruation and pregnancy, phimosiectomy, etc) that they experienced or would experience due to their biological sex. A part of them asked the questions based the misconceptions and used the slangs or ‘××’, ect. to name the male or female genitalia. The male students wanted to know the female genitalia. The participants' understanding of gender reflected the sexism of our society, but they didn't accept and had doubts about the dual, hierarchical structure of that. The students, especially female seemed to be powerless to the harms of the sexual violence. Girls had much interest in their appearances and complained to our sexual culture that women comforted and served men. The participants had the dual perspective that the sexuality as respects of physiologic phenomenon was considered as natural but that as related to heterosexuals was as negative. And they deemed that men's sexual drive was stronger than women's and was difficult or was not able to be inhibited. They had much interests in homosexuality but reflected the negative attitude to that in our society. Adolescent felt wonder why the expressions of sexuality of adult were permitted but theirs were not. Lastly, a part of boys substituted querying the sexuality of animals for asking that of human. Maybe it was because of the embarrassment to talk about human's sexuality directly. The teenaged participants understood that the sex had not only the biological meanings but also the sociocultural meanings. They regarded the sex as natural and wanted to know it, but they had conceptions that it was difficult and embarrass to talk about it openly and directly.