Background: Tuberculosis/Human immunodeficiency virus (TB/HIV) co-infection is bidirectional and synergistic which mainly affects interventions that have been taken on the area. Tb patients co-infected with HIV have poorer treatment outcome as compared to non-co-infected patients. There is limited information regarding successful TB treatment outcomes and its associated factors; a reason that this study was planned to investigate. Methods: An institution based cross sectional study was carried out from July 2010 to January 2016. Data were abstracted from patients' medical chart using data abstraction format. The completeness of the data was checked and cleaned manually. Then, it was entered and analyzed by using SPSS version 20.0. Bi-variable and Multi-variable logistic regression model was fitted to identify factors associated with successful Tb treatment outcome. Significance was obtained through adjusted odds ratio with its 95% CI and a p < 0.05. Results: Successful TB treatment outcome among TB/HIV co-infected patients in Gondar University Hospital was 77.3% [95%CI 72.6-81.9]. Being residing in outside the Gondar town [AOR = 0.44, 95%CI: 0.25-0.80], having less than the mean baseline weight (<43.7 kg) at initiation of TB treatment [AOR = 0.51, 95% CI: 0.29-0.89], being in the bedridden condition [AOR = 0.23, 95% CI: 0.1-0.23], and experiencing anti-TB treatment side effect [AOR = 0.35, 95% CI: 0.12-0.98] were the factors that resulted the patient in treatment failure. Conclusion: Successful Tb treatment outcome among TB/HIV co-infected patients was lower than the target set by Global Plan to Stop TB 2011-2015. Strengthening collaborative TB/HIV management activities that would trace the identified factors shall be recommended to increase successful treatment outcome of TB.