Background: Adolescence is the last chance for breaking the intergenerational cycle of malnutrition and poor health, despite it is a neglected age groups. Therefore, this study is designed to assess the prevalence and the factors associated with under nutrition among school adolescents in Hageremariam district, central Ethiopia. Methods: Institution based cross-sectional study was conducted on 381 adolescent living in Hageremariam district, from June 10-26/2019 using multistage sampling technique and structured pretested questionnaire. Anthropometric indices (height-for-Age and BMI-for-Age) were generated by using WHO Anthro plus software. Bi-variable and multivariable logistic regression were carried out to compute OR with 95%CI. Statistical significance was declared at p <0.05. Results: The prevalence of stunting and underweight were 18.6% and 18.9% respectively. Factors identified for stunting were family size > 5, having no latrine in the compound and adolescent food insecurity. Having illness in the last 2weeks preceding the study, having no latrine in the house-hold and hand washing practice after toilet were the predictors identified for underweight. Conclusion and recommendation: The study revealed that nearly one in five school adolescents are undernourished. Family size, adolescent food insecurity, hygienic practice /sanitation, and having illness in the last two weeks preceding the study were found to be an important factor of under nutrition. Thus, nutritional intervention program would better to consider these factors in the study area.