Background: Malnutrition is the condition that results from an imbalance between dietary intake & requirements. Many factors can cause under nutrition, like human immune virus infection and it commonly affects all age groups in a community, but infants and young’ children are the most vulnerable group because of their high nutritional requirements for growth and development. Objective: To assess magnitude of under nutrition and associated factors among HIV/AIDS infected children attending in HIV- care at Saint Paul's Hospital, Addis Ababa, Ethiopia; 2021. Methods: Institutional based cross-sectional study was conducted among 394 HIV/AIDS infected children age 2-10 years attending HIV- care in Saint Paul's Hospital Millennium Medical College from February 1/ 2021 to March 1/2021. Systematic random sampling technique was used to select the study participants and face to face interview, anthropometric measurement and clinical records review was employed to collect data. Data were entered into Epi-info.7 and analyzed using SPSS version 25. Binary Logistic regressions analysis was computed and variables with a p-value of < 0.25 in the bivariate analysis were included in to multivariable analysis and those with P-value <0.05 were considered as statistically significant. Result: Out of 405 sampled HIV positive children, 394 were responded to the questionnaires giving a response rate of 97.2%. Magnitude of under nutrition was 62.2% [95% CI: (57.4, 66.9)] of which 44.9%, 36.3% and 24.6% were stunted, underweight and wasted respectively. Being age 61-120 month, low educational status, eating not enough food as family wants, poor verity diet, recurrent oral lesion, low CD4 count and early age of diagnosis were more likely associated with stunting. Low monthly income, not receiving supplemental nutrition and diarrhea lasting for more than 2weeks were positively associated with wasting. Being male, being age 61-120 month, occupations other than a house wife, low monthly income, diarrhea lasting for more than 2weeks, early age of diagnosis and advanced disease stage were associated with underweight. Conclusion: In this study magnitude of under nutrition was very high. Early age, low educational level of mother, being male, low income, mother occupations other than a house wife, not eating enough xi and verity diet at home, recurrent oral lesion, low CD4 count, diarrhea, not receiving supplemental nutrition and advanced disease stage were identified as factors of under nutrition. Policy makers, hospitals and health care providers should give attention on addressing these factors to decrease nutrition related mortality and morbidity among HIV infected children. Key word: Under nutrition, human immune virus, children, Saint Paul's Hospital, Ethiopia