Background : Free sex is a relationship that is carried out by men and women without marriage ties. Premarital sexual behavior is a sexual activity that involves two people who like each other or love each other, which are carried out before marriage. Data from the Ministry of Health of the Republic Indonesia (2011) show the number of adolescents aged 10 – 19 years is around 43 million (19, 61 %) of the population, 1 million male teenagers (5 %) and 200 thousand female adolescents (1 %) openly state that they have already have sexual relations. There is also the impact of having free sex before marriage is pregnancy outside marriage, is seen as not good in the midts of society, the possibility of contracting sexually transmitted diseases, and psychological effects that arise such as lack of selfconfidence, feel have made a big mistake and sin, fear of pregnancy after sexual intercourse, for women considered to be unholy or not virgin, for men considered to be no longer a virgin, have problems in marriage and tend to have a strong bond between couples. Research Purposes: To find a picture of the level of knowledge to nursing students level 1 aged 18 – 21 years about free sex at one of the private universities in Tangerang. Research Methods: This research uses Quantitative methods with descriptive design, the sampling technique uses purposive sampling. This research uses a questionnaire instrument. The study population numbered 337 people and with the Slovin formula determined a minimum number of samples that is 161 respondents. The data analysis technique uses is univariate analysis. This research was conducted in February – Mei 2020. Results: Shows that the majority of respondents have sufficient knowledge about free sex with 77 respondents (46,10 %), less knowledge with 65 people (38,92 %), and respondents with good knowledge with 25 people (14,98 %). Recommendation: For further research is expected to be able to examine the attitudes and behavior of adolescents in free sex.