Context: Sexual intercourse during adolescence period is unprotected and it has a lot of health risks and physical and psychological hazards. Meanwhile there is no study on the prevalence and associated factors of premarital sexual practice in the study area. Aims: The aim of this study is to assess the prevalence of premarital sexual practice and associated factors among Alamata high school and preparatory school adolescents in 2013. Settings and design: The study was conducted in Alamata, Tigray which is the northern Ethiopia. There is one high school and one preparatory school. The study design was cross sectional. Methods and materials: All regular students attending their school in the year 2013 were included and multi stage sampling method was used. A total of 520 participants were included. Data were collected using self-administered questionnaire. Statistical analysis used: Data entered using EpI Info version 3.5.1 analyzed using SPSS version 16. Descriptive, bivariate, and multivariate logistic regression analysis were used Result: A total of 493 participants give complete response. One hundred four (21.1%) of adolescent have had premarital sexual practice. Associated factors were age>18 years (AOR=12, 95%CI=3.97, 36.54), urban resident (preventive) (AOR=0.23, 95%CI=0.07, 0.75), having a positive attitude towards premarital sex (AOR=3.07, 95%CI=1.19, 7.91), having a boy/girlfriend(AOR=3.33, 95%CI=1.39, 7.99), peer pressure (AOR=7.33, 95%CI=2.97, 18.09), and watching sex movies (AOR=7.98, 95%CI=2.55, 24.93). Conclusion: Prevalence ofpremarital practice was high. Therefore, community and/or school health interventions are needed to reduce the premarital sexual practice among school adolescents to prevent sexually related health problems.