Background: Human immune deficiency virus is one of the greatest challenges to global health faced by the medical profession. HIV infection is particularly aggressive in children without access to treatment in that, more than half of HIV-infected infants die before the age of 2 years. Pediatric Malnutrition is estimated to contribute to more than 20 million childhood morbidity and mortality. HIV and malnutrition are common comorbidities in low and middle income countries. Objective: To assess the prevalence and associated factor of underweight among children age 5-15 years living with Human immune deficiency virus in Bahir Dar city North west ,Ethiopia. Methods: Institution based cross-sectional study was conduct from March 15 /2019 to April 15/ 2019. Sample size was 226 and sampled by simple random sampling technique. Data were collected using semi structured questionnaire. Data was entered by Epi Data version 7 and analyzed by SPSS version 20. Bivariate and multivariate logistic regression analysis were performed to identify factors associated with underweight. Adjusted Odds Ratios (AOR) with the corresponding 95% Confidence Interval (CI) were used to show the strength of associations, and variables with P-values of < 0.05 were considered statistically significant. Result ;. In this study, the overall prevalence of underweight was 35.9 % . According to the multivariable logistic regression analysis , Clinical staging , AOR 2.93, CI (1.51,5.70) , Duration of HARRT , AOR = 8.47 CI: (1.77,40.49) ,Recent viral load AOR= 4.54 CI; (1.61 ,12.76) and were significantly associated with underweight. Conclusion: Prevalence of under nutrition was high from EDHS . Family size ,Clinical staging , duration of HARRT , Recent viral load and Hemoglobin count were associated with underweight . Therefore, interventions on children HIV care management is appropriate to manage the problem of underweight.