Introduction: For nutritional assessment of HIV-positive adults and adolescents, it is recommended to takeWeight, Height and MUAC (for pregnant and lactating women and/or adults who cannot stand straight) in every contact, such measurements increase the waiting time and inconvenience(1). So that a simple and yet useful anthropometric measurement is needed to screen malnutrition for an immediate nutrition support in developing countries. Objective: To assess the sensitivity (SN) and specificity (SP) of mid-upper arm circumference (MUAC) in detecting under nutrition of adult people with HIV who have a follow up at ART clinic. Methods: A cross-sectional study was conducted from January to July 2013 among adults with HIV (PLHIV), who have follow-up in three selected Governmental Hospitals of Addis Ababa city administration. A total of 594 study subjects were selected using a multi stage cluster sampling technique. MUAC, Height and weight were measured for all subjects and BMI was calculated as Wt/Ht 2 . Receivers operating characteristic curve analyses were undertaken to discover the most suitable cut-off values of MUAC for both Men and Women. Results: The prevalence of chronic energy deficiency (CED) based on BMI (BMI <18.5) was 39.4% for males and 24.21% for females, while based on MUAC <230 mm for male was 48.4% and < 220 mm for female was 35.7%. The sensitivity and specificity of MUAC for Female with optimal criteria of ≤224 for females were 92.86 and 87.07, whereas for Males MUAC with optimal criteria of ≤234 were 94.85 and 68.46, respectively. The area under curve (AUC) was 0.93 (p<0.001). A MUAC value of ≤239mm was identified as the best cut off-points to identify CED (BMI <18.5) with SN and SP of 96.43% and 72.62%, respectively. The PPV was 56.6(95%CI=89.9 to 99.3) with the highest NPV 98.2 (95%CI=66.8 to 77.9), thus having the highest Youden Index of 0.21. A MUAC value of ≤242mm was identified as the best cut off point to identify CED (BMI <18.5) with SN and SP of 96.91% and 58.39%, respectively. The PPV was 46.3(95%CI=91.2 to 99.4) with the highest NPV 98.1 (95%CI=50.0 to 66.4), thus having the highest Youden Index of 0.32. Conclusion: a MUAC value of close to 242 mm for male and 239mm for female seemed to be appropriate as a simple and efficient cut-off point for the determination of under nutrition in adult PLHIV.