Introduction: Human immunodeficiency virus/ Acquired immunodeficiency syndrome (HIV/AIDS) is one of the major public health Problem worldwide and its epidemic is occurring in populations where malnutrition is already endemic. Ethiopia is among the countries most affected by under nutrition and HIV epidemic in the region. The objective of this study was to determine the prevalence of under nutrition and associated factors among adult people living with HIV/AIDS and on ART in Achefer Woreda, North West Ethiopia.Methods and Materials: Institution based cross sectional study design was used and conducted from May 01–30/2015. Systematic random sampling technique was used to select the study subjects and the data was collected using clinical assessment, measurements and interviewer administered questionnaire. To identify independent predictors of under nutrition of adult people living with HIV/AIDS and on ART, we performed multivariable logistic regression analyses using SPSS version 20 with CI of 95% at p-value < 0. 05.Result: Three hundred fifty HIV/AIDS on ART patients were included in the study. The overall prevalence of under nutrition was 26.9% and females were most affected 57 (18.1%). Anti-retroviral treatment duration of 6-11months and 12–24 months (AOR = 4.72, 95% CI, 1.10-20.35) and (AOR = 6.93, 95% CI, 1.614–29.754) respectively, WHO Stage two and three (AOR = 3.01, 95% CI,1.061–8.534) and (AOR = 12.56, 95% CI, 4.27–36.99) respectively and dietary counseling (AOR = 0.20, 95% CI, .05-.78) were significantly associated with under nutrition.Conclusion and Recommendation: Undernutrition was high in PLWHA and on ART. ART duration, WHO clinical staging, presence of eating problem and dietary counselling were the predictors for under nutrition of HIV patients on ART. Only increasing access to ART can’t solve problem of under nutrition, therefore nutrition therapy and support, site expansion also as an accessory to the initiation of ART should be considered.