There is a growing concern about premarital sex in today's society and research. A cross-sectional survey of unmarried people was conducted in Hanoi, Da Nang and Can Tho to determine the premarital sex situation and risk factors. A questionnaire was completed by 1508 people who are 15 - 49 years old. The prevalence of premarital sex in three provinces is 8.1 percent, 13.1 percent in male and 3.4 percent in female. People who are male, over 25 years old and agree with having premarital sex are risk factors in association with premarital sex. It is necessary to have more research to identify risk factors and protective factors related to premarital sex in Vietnam.