Introduction: Orphans are potentially at greater risk of malnutrition because they are more likely to be extremely poor, receive less medical and social care. They comprise almost 12% of Ethiopia’s total child population and half of children below the age of five years are stunted. Inadequate funding to support programs designed for the children, shortage of trained personnel, inadequate skills training that resulted in long care in orphanage challenges in the in Ethiopia. However, there is a little information about the prevalence of under nutrition and associated factors among under five orphan children. Thus the objective of this study was to assess the prevalence of under nutrition and associated factors among under five Orphan children in orphanages in Addis Ababa, Ethiopia. Methods: Institution based cross-sectional study was conducted in Addis Ababa. Simple random sampling technique was employed to recruit a total of 275 orphan children by using lottery method. Interviewer administered questionnaire was used to collect data by data collectors and the child’s nutritional status were assessed anthropometrically. Data was entered using Epi data version 3.1 and analysis was done by WHO Anthro version 3.2.2 and SPSS version 23. Multivariable logistic regression analysis was performed to identify determinants of under nutrition at a p-value of less than 0.05. Results: In this study the magnitude of wasting, stunting and underweight among orphans were 11.1% (95% CI: 7.7-15.1), 45.8% (95% CI: 39.9-52), and 25.5% (95% CI: 20.3-31) respectively. Orphan who had illness is (AOR = 2.23; 95% CI: 1.41, 12.73), children who received less than 3 meals per day (AOR = 2.11; 95% CI: 1.58, 7.71), and children who was not vaccinated (AOR = 2.86; 95% CI: 2.07, 11.61) were significantly and positively associated factors with stunting. Children who were not vaccinated (AOR = 2.04; 95% CI: 1.29, 9.21) and had inadequate dietary diversity score (AOR =1.32, 95%CI: 1.16, 12.65) were significantly and positively associated with wasting and underweight, respectively. Conclusion The prevalence of under nutrition was found to be high among orphan children .Meal frequency and vaccination status were factors associated with stunting. Vaccination status and dietary diversity score were factors associated with wasting and underweight, respectively. Therefore, improving meal frequency, dietary diversity, immunization coverage and early treatment during childhood illness will be very important. Keywords: Under nutrition, Orphan children, Addis Ababa, Ethiopia