The bidirectional relationship between the twin epidemics of Tuberculosis (TB) and Human Immunodeficiency Virus (HIV) causes major global health challenges in the twenty-first century. TB-HIV co-infected people are facing multifaceted problems like high lost to follow up rates, poor treatment adherence, high TB recurrence rate, and high mortality risk. Our objective was to assess the outcomes of TB treatment and associated factors among TB-HIV co-infected patients in Harar town, Eastern part of Ethiopia, 2018.A retrospective study was conducted among systematically selected 349 TB/HIV co-infected patients who registered from 2012 to 2017 in two public hospitals in Harar town. The data were collected through document review by using a pre-tested structured data extraction checklist. The data were analyzed using SPSS Version 21. Bivariate and multivariate logistic regression were determined at 95% confidence intervals.Among the 349 TB/HIV co-infected patients included in the study, 30.1% were cured, 56.7% had completed their treatment, 7.7% died, 1.7% were lost to follow up, and 3.7% were treatment failure. Overall, 86.8% of the TB-HIV co-infected patients had successful TB treatment outcomes. The patients who were on re-treatment category (AOR = 2.91, 95% CI: 1.17-7.28), who had a history of opportunistic infection (AOR = 3.68, 95% CI: 1.62-8.33), and who did not take co-trimoxazole prophylaxis (AOR = 3.54, 95% CI: 1.59-7.89) had 2.91, 3.68, and 3.54 times higher odds of having unsuccessful TB treatment outcome than their counterparties, respectively. The chance of unsuccessful TB treatment outcome was 4.46 (95% CI: 1.24-16.02), 5.94 (95% CI: 1.87-18.85), and 3.01 (95% CI: 1.15-7.91) times higher among TB/HIV patients in stage 2, 3 and 4 than those in stage 1, respectively.The overall rate of the success of the TB treatment among TB-HIV co-infected patients in this study was higher compared with many previous studies. TB/HIV patients with a history of previous TB treatment, smear-positive pulmonary TB, late HIV stage, history of opportunistic infection and not being on co-trimoxazole prophylaxis therapy were at a high risk of getting poor treatment outcomes.