Background: HIV/AIDS is one of the most devastating infectious diseases affecting humankind overall the world and its impacts goes beyond public health problems. The primary goal of this study was to analyze predictors for CD4 cell count and hemoglobin level with time to default jointly for HIV positive adults under ART treatment at University of Gondar Comprehensive Specialized Hospital. Method: This study was conducted at University of Gondar comprehensive specialized hospital (UGCSH), Gondar, Ethiopia by using a retrospective cohort follow up study design from the medical records of 403 randomly selected adult HIV positive out patients who enrolled treatment from September 2015- march 2022 G.C. The source of data in this study was secondary data obtained from patients chart and the data were explored by using individual and smooth profile plots, Kaplan-Meier plots, and log-rank tests. A bivariate linear mixed-effects model, Cox PHs model and Bayesian joint models were employed to get wide-ranging information about HIV/AIDS progression. In the Bayesian joint model, longitudinal and survival sub models were linked by their current value and slope parametrization. Result: From a total of 403 HIV positive adults, about 44.2% were defaulted from therapy and the rest were actively followed ART treatment. The overall mean and median estimated survival time of patients under study was 44.3 and 42 months respectively. The estimate of the association parameter for the current true value of Hgb level and CD4 cell count indicated that for a one unit increase in the current true value of Hgb level and CD4 cell count, the rate of time to default from treatment had increased by 60.6% and 0.35% respectively. The predictor’s platelet cell count, BMI, WHO clinical stage, and tobacco addiction were jointly associated with CD4 cell count, Hgb level and time to default from treatment. Conclusion: A patient who had a good visiting time, low bassline platelet cell count, low BMI, shows a high chance of increasing CD4 cell count and Hgb level, consequently, patients to experience more hazard of defaulting. In addition, WHO clinical stage, and tobacco addicted patients were with low CD4 cell count, Hgb level and time to default from treatment Keywords: CD4 Cell Count, Hgb Level, Time to Default, Bayesian Approach, Joint Model