BACKGROUND: The age of adolescent is a phase of growth and development in the life process that need adequate and proper quality food to meet the nutrient requirement for their physical and mental growth and also development of reproductive maturity. Need of food during adolescence are influenced by the puberty age connected with increased growth rate and changes in body composition. OBJECTIVE: The objective of this study was to asses under nutrition of adolescent girls and associated factors in Lay armachiho district high schools, Northwest, Ethiopia, 2018. METHOD: Institutional based cross sectional study was conducted from February to March to assess under nutrition and associated factors in adolescent. The sample size was calculated based on the single proportion formula using the prevalence of height for age in adolescents 31.5%. The final sample size was 366 adolescent girls. The data were collected by using structured questionnaire and Anthropometric measurement and the study employed systematic sampling technique to select adolescents. Data was entered in to Epi and transferred to SPSS for analysis. WHO Anthroplus software was used to calculate BMI for age z-score and height for age z-score. The data would be analyzed using bivariate and multivariate logistic regression. The degree of association between dependent and independent variables was assessed using odds ratio with 95 % confidence interval and variables with p-value ≤ 0.05 are considered significant. Result. The prevalence of wasting and stunting was 10.4 % and 23.2 % respectively. Family size [AOR = 0.45 (0.21, 0.93), dietary histories [AOR = 2.1 (1.02, 4.31)] and water source [AOR 0. 39 (0.19, 0.84)] were significantly associated with wasting whereas meal frequency [AOR = 2.2 (1.27, 3.81)], marital status [AOR = 2.19 (1.21, 3.97)], residence [AOR = 1.87 (1.08, 3.23)] and disease histories [AOR = 1.78 (1.06, 2.98)] were significantly associated with stunting. Conclusion and Recommendation: wasting and stunting problems are found among adolescent girls in Layarmachiho district high schools. District health office provides strategies to improve the nutritional status of girls and given much attention. Key