Introduction: Post term births is major determinants of neonatal mortality, including short- and long-term morbidity. In developing countries, where post-term birth is disproportionately common, the magnitude and underlying causes are not well understood, and evidence is required to design appropriate interventions. In Ethiopia the level of post term birth proportion was not studied, except few. Determining the proportions and associated factors for post-term births in Bahir Dar city is rational for appropriate intervention. Objective: The aim was to determine proportion of post term deliveries and associated factors among mothers who gave birth in Bahir Dar city administration governmental hospitals, Northwest Ethiopia, 2020. Method: Institutional based cross-sectional study was conducted among 423 systematically selected mothers who gave birth in Bahir Dar city administration governmental hospitals, From February to March 2020. Data entry and analysis was made using by epi-data version 3.1 and SPSS version 21 respectively. In binary logistic regression of dependent and each independent variables with P value ≤0.2 were included in multi-variable logistic regression and P-value ≤0.05 in multi-variable logistic regression were considered as significantly associated with post term birth. Result: The proportion of post term was found to be 14.4% with [95% CI=11.1%-17.5%]. Rural residence [AOR=4.06 (95% CI=1.563,10.528)], Far from nearby health facility [AOR=5.250 (95%CI=2.489,11.071)], null parity[AOR=2.368 (95%CI=1.113,5.038)], not having ANC [AOR=7.218 (95% CI=1.108,47.024)], number of ANC [AOR=4.383 (95%CI=1.609,11.936)], family history of post term birth [AOR=2.417 (95%CI=1.075,5.436)] and no knowledge about BPCRP [AOR=2.604 (95%CI=1.215,5.584)],were significantly associated with post term birth. Conclusion: The proportion of the post term delivery was high. Factors associated for this result were rural residence, far from nearby health facility, null parity, not having antenatal care, number of ANC visit, family history of post term birth and lack of awareness about Birth preparedness and complication readiness plan. To decrease this proportions of post term birth; further efforts expected from maternal health care providers to increase awareness of mothers about importance of BPCRP, to have ANC follow-up and sustain their follow-up till their birth will reduce the problem so far. Key word: Associated factors, Post term Birth, Proportion and Bahir Dar, Ethiopia