Adolescence has so many problems one premarital sexual behavior. This is because changes in physical, psychological and intellectual encourage teenagers in risky behavior. The purpose of this study was to observe the effect of premarital sex education with peer education methods to increase knowledge and attitudes about premarital sexual behavior in SMK "XY" Padang. The type of this research is Pre-Experiment with One Group Pretest-Posttest design. Data collection was collected in May to July 2019. The population in this study was tenth grade Vocational school students in Padang area on 2018/2019 academic year. It's about 531 people. The amount sample of this research was 60 subjects. The sample was taken by simple random sampling technique. Data processing was carried out by Wilcoxon Test (p <0.05) using SPSS 17 software. The results showed that there was an improve in the knowledge and attitudes of adolescents about premarital sexual behavior aftergiven education by the peer education method. The results of the Wilcoxon test analysis obtained p value of the effect of premarital sex education with the peer education method on increasing adolescent knowledge about premarital sex behavior (p = 0,000). There is the influence of premarital sex education with peer education methods to improve the knowledge and attitudes of adolescents about premarital sexual behavior in Vocational School "XY" Padang in 2019