A descriptive study was conducted at Mahidol Witthayanuson school in Salaya, Thailand during February, 1999 to determine the sexual behaviour of the Thai school adolescents . A total of 196 (65 male and 131 female) school adolescents were given a self administered questionnaire in order to obtain information on sexual behaviour. The study revealed that only 25 adolescent students (about thirteen percent) had sexual intercourse experience. But about fifty percent of the respondents had other different forms of sexual experiences including masturbation, kissing cheek, kissing lips, massage and touching sex organs. Various socio-demographic characteristics namely age, gender and dating have strong relationship with (p-value .<0.05) sexual behaviour. Regarding sexual knowledge of adolescents, almost equal percentages (eighty one and eighty nine) of male and female adolescents had same knowledge about sexuality. Regarding reproductive health, only 6 female respondents have multiple sex partners with having 1 or > 1 menstrual regulation experiences. Knowledge about contraception was almost fair (about sixty seven percent ) indicated that, almost all adolescents were conscious about consequences of not use of contraceptives. The information from the results of the study could help to formulate a practical strategy in future plan of Thai national sex education program with the ultimate goal to reduce the adolescents health problems, improving the quality of life of Thai people.