Background: Today, adolescents are recognized as groups with special health-relatedrnvulnerabilities .They are exposed to various risks such as unprotected sex, early marriage, earlyrnpregnancy, sexually transmitted infections (STIs) and HIV/AIDS. Many adolescents are sexuallyrnactive. These sexual activities put adolescents at risk of various reproductive health problemsrnincluding STI and HIV/AIDS.STIs have occurred mostly among young people, with the highestrnreported rates found among those aged 15-24 years.rnObjective:rnThe major objective of this study is to assess the sexual behavior and thernknowledge of STI and HIV/AIDS among high school adolescents.rnMethod: A cross-sectional study was conducted on 562 adolescents using a pre testedrnquestionnaires from March to April 2014, among two high schools, in Jardega Jarte woreda,rnHoro Guduru Wollega zone, west Oromia regional state.rnResult: From the total respondents, 38.6 % ( 53% of male and 47% of female) of adolescentsrnhad experienced sexual activity. Among those, only 46.6% of them had used condom the first timernthey had sexual intercourse. The mean age at first sexual intercourse was 16.24 (16.24 Â± 0.682)rnfor male and 15.40 (15.40Â±0.796) for females. From all sexually active adolescents, 92.2%rn(97.2% of boys and 86.7% of the girls) had their first sexual intercourse between the ages of 15rnand17 years.rnThe reasons cited for the initiation of their first sexual intercourse were fall in love (37.9%), hadrndesire, 74 (36%), peer pressure, 30 (14.5%), were drunk, 12 (5.8%), to get money or gifts, 5rn(2.4%), rape, 4(1.9%), and got married, 2 (0.9%).rnAmong all adolescents, 45.7% of students have heard about sexual intercourse other than vaginalrnwhile 54.3% of students have not heard.rn98.9% of the respondents claimed to have ever heard about sexually transmitted diseases (STIs),rnof which, HIV/AIDS (96.6%), gonorrhea (75.8%), Syphilis, (74.3%) and Chancroids (40.1%),rnwere the most commonly known types of STI.rnThe most commonly cited sources of information for HIV/AIDS were media, such as radio andrnTV, (78.3%), school clubs,(74.5%) and followed by school teachers, (62.9%).rnThe majority of the adolescents mentioned the modes of HIV/AIDS transmission to be throughrnunprotected sexual intercourse (96.4%), sharing contaminated instruments (81.8%) followed byrnfrom mother to child (68.4).rnConclusions and recommendations: from this study, one can conclude that substantialrnproportion of the adolescents ever had sexual intercourse was higher. Moreover adolescents arernpracticing risky sexual behaviors such as unprotected sex, multiple sexual partners, and sex atrnearly age. Therefore, an integrated effort needs to be initiated to address such adolescentsâ€™ sexualrnand reproductive health problems through establishing and strengthening school anti AIDS clubs,rnproviding in service training for teachers on adolescent reproductive health and establishingrnyouth centers