Background: Malnutrition among school adolescent girls is a serious concern in low income countries. In addition to the immediate negative health and development effects undernutrition at this age can substantially reduce educational achievements. However, data indicating the magnitudeandassociatedfactorsamongschooladolescentgirlsisscarce. Objectives: The objective of this study was to assess the prevalence of undernutrtion of school adolescentgirlsandassociatedfactorsinAradasubcityAddisAbabaEthiopia. Methods: School based cross-sectional study was conducted among randomly selected 420 adolescent school girls in two private and two government schools of Arada sub city Addis Ababa. Data were collected using a structured and pretested self-administered questionnaire and taking anthropometric measurements. Measurements were taken following recommended standard procedures. Anthropometric data were processed following WHO recommendations. Adolescent girls were identified as stunted if height-for-age Z score is <−2 standard deviations of the median of the reference population and thin if BMI for age Z score is <−2 standard deviations. Binary logisticregressionwascarriedouttodeterminefactorsassociatedwithunder-nutrition. Results: Findings of this study revealed that the prevalence of stunting was 3.1% (95% CI: 1.3%, 4.9%). Stunting was more common among adolescent girls being from family members were less than five [(AOR=4.06; 95% CI: (1.30- 12.68)] than those whose family member greater than five. Moreover, thoseadolescents bornfromhousewife mother were less likely [(AOR =0.21;95%CI; (0.05- 0.93)] to be stunted than those adolescents born from working mothers. The prevalence of thinness was 4.3 % (95% CI: 2.2%, 6.3%). School adolescent who reported as having pocket money were less likely [(AOR=0.31; 95% CI :( 0.11- 0.88)] to be thin than those who never have pocketmoney. Conclusions and recommendations: Undernutrition among studied adolescent girls was not uncommon problem. Adolescent girls from less family member, working mothers and not having pocket money were more affected by undernutrition than others. School based nutritional interventionscanbeusefultoensurechildren'sfulleducationalbenefitsandachievements.