Introduction- Birth interval is the time between two consecutive live birth that allow the mother to adequately recover from the nutritional deprivation and health issues related to pregnancy and lactation. Among many, one of the key measures to promote the health of mothers and their children is adhering to optimal birth interval. However, in developing countries including Ethiopia women still have short birth interval that also has economic impact on nation’s development therefore in general identifying its determinants of birth interval is important. Objective- To assess duration of birth interval and its associated factors among mothers who gave birth at government health facilities of Bahirdar city, Amhara regional state, Ethiopia, 2021. Methods-Institutional based quantitative cross sectional study done on 576 mothers who gave birth at respective health facilities. Systematic sampling technique used to select participants and they were interviewed by using structured questionnaire from April 1- May 31 2021 G.C. After data collection, data was coded and entered to Epi data 3.1 and exported to SPSS version 20 then multinomial logistic regression was done. A p-value of < 0.05 considered as cut off point to assess significance of associations in the multivariable analysis. Result- According to this study 58% (95% CI, 54-62) of the participants have short birth interval and 12.8 %( 95% CI, 10.2-15.8) of them have long birth interval. Being in tertiary educational level AOR= 2.32 95% CI (1.776-3.519), having a girl child previously [AOR=2.222, 95% CI :( 1.563, 3.159)] and also not having PNC follow up (AOR=3.097, 95% CI: (2.451, 3.912)) were the main predictors of short birth interval and vaginal mode of delivery [AOR= 0.012, 95% CI: (0.004-0.037)] is the factor associated with long birth interval. Conclusion and Recommendation -Majority of the participants have short birth interval. Therefore to achieve optimum/adequate birth interval methods of providing information and education targeting predictor variables should be improved. Key Words: Bahirdar, Birth interval, Long, Suboptimal/Short