The number of Indonesian adolescents practicing sex before marriage is relatively low compared to the rate in other countries (less than 6.5%). However, this issue should be given attention because the rate increased from 5% to 6.4% between 2002 and 2007. Therefore, the main aim of this study is to analyze factors influencing premarital sexual intercourse among in-school late adolescents in Indonesia. This study uses secondary data from the research of the Indonesian Young Adult Reproductive Health Survey (IYARHS) 2007. The study population of 6,792 in-school late adolescents aged 15-19 years was selected from a total of 19,311 unmarried Indonesian young adults that were interviewed. The findings illustrate that 2.4% of the in-school late adolescents have engaged in premarital sexual intercourse. Evidence indicated that sex, age, place of residence, knowledge of sexual reproductive health (SRH), knowledge of contraceptive methods, source of SRH information, attitude towards acceptance of premarital sexual intercourse, relationship status defined by having a boyfriend or girlfriend, risky consumption behaviors, and peer influences have a significant association with in-school adolescents that have experienced premarital sexual intercourse. This study suggests that strengthening SRH education programs in every sector in the community for adolescents should be scaled up to increase their knowledge and make them less curious about practicing premarital sexual intercourse.