The purpose of the present study was to assess the sexual behavior of adolescents in single -rnsex and co-education schools particularly those who are attending class in preparatoryrnschools at Addis Ababa. It specifically examines the relationships among sexual behavior,rncontraceptive use and sex associated risks of adolescents. The study has used a mixed methodrnresearch design. A total of 320 participants were selected using stratified random samplingrntechniques coupled with purposive sampling. More specifically, while the one single sexrn(boys only) school and informants of FGD and KII were purposively selected, the other threernschools were selected through simple random sampling techniques.The respondents of thernquestionnaire scales were also selected through stratified random sampling. Moreover, bothrnquantitative and qualitative data were generated from using both quantitative (BSASrnquestionnaire scales) and qualitative (FGD and KII) methods. In order to address the basicrnresearch questions, data were analyzed using Pearson correlation coefficients, Independentrnt-test, and ANOVA statistical techniques. Accordingly, the current study generally revealedrnseveral important findings about sexual behavior, contraceptive use and sex associated risks,rnand about their relationships. First, most adolescents become sexually active at around thernage 16 and 17. It was also found that the driving forces for initiating sexual intercourse are:rnpeer pressure, falling in love and the need to explore and experiment. Second, most of thernadolescents do not use contraceptives when having sexual intercourse. Third, manyrnadolescents are facing sex associated problems such as STDs, unwanted pregnancy andrnabortion. Fourth, a negative correlation was obtained between sexual behavior scores andrnsex associated risk factors scores. Fifth, a strong relationship has been found betweenrncontraceptive use and school type; Sex Associated Risks and grade level; Sexual Behaviorrnand the age of adolescents. Finally, based on the results of the current study, the followingrnrecommendations were forwarded:provision of SRH training and contraceptives likerncondoms in the school settings; and more researchers should be conducted to see the impactrnof school types on adolescentsâ€™ sexual behavior so as to provide data to policy makers and allrnstakeholders in the education sector.