Background: Bloody diarrhoea is defined as diarrhoea with visible or microscopic blood in the stool and cause severe complications such as septicemia, anemia and Zink deficiency particularly in young children. The 2016 Ethiopian Demographic Health Survey and study conducted in seven low income and middle income countries including Ethiopia showed bloody diarrhea still to be the higher cause of morbidity and mortality in under-five children. So the aim of this study was to identify the determinants of acute bloody diarrhea among under-five children in the study area. Methods: Facility based unmatched case-control study design was conducted from March 2019 to May 2019 on 312 under five patients. Data were entered in to Epi-Data version 3.1 and exported to Statistical Package for Social Science Version 20 for analysis and Adjusted Odds Ratio with 95% confidence interval was computed and p-value less than 0.05 in multi-variable logistic regression was considered as significantly associated with the outcome variable. Result: According to the study the determinants of acute bloody diarrhea in children were using water for drinking from unprotected water source(AOR:3.2, CI: 1.52-6.74), human Co-inhabitation with animals (AOR: 2.5, CI: 1.37-4.51), not disposing infants’ or children’s feces into latrine (AOR: 1.2, CI: 1.02-3.57), under nutrition (AOR: 3.8, CI: 1.66-8.76) and family size of households ≥5 (AOR: 2.1, CI: 1.21-3.72). Conclusion and recommendation: In this study majority of the determinants of acute bloody diarrhea among under-five children in the study area were environmental related factors such as water for drinking from unprotected source. To reduce the transmission of acute bloody diarrhea in children, the local Government, other stakeholders and partners should invest and work harmoniously for improvement of safe water supply and other hygienic practices. Key words: Acute bloody diarrhea, Basoliben woreda, determinant, under-five year’s age