Introduction: Globally sever acute malnutrition affects 17 million under five children. More than one quarter of those children lived in Africa. In Ethiopia about 3% of children were severely wasted. Sever acute malnutrition were seen persistent over the past 15 years .To implement an effective intervention, that can reduce this high burden sever acute malnutrition; it is essential to understand factors predispose to it.. Objective: To identify the determinants of severe acute malnutrition among under five children in Felege Hiwot and Debre Tabor Referral Hospitals, North West Ethiopia. Methods: Institution based age matched case control study was conducted on 104 cases 208 controls. Cases were children aged 6-59 months who had sever acute malnutrition and controls were 6-59 months of children without malnutrition (weight for height ≥ 90percentile, MUAC>12 cm and without bilateral pitting edema). Bivariate and multivariate analyses were done using conditional logistic regression in order to determine factors associated with severe acute malnutrition. P-values less than 0.05 were used to identify determinants of SAM. Results: A total of 104 cases and 208 controls were included in the study with overall response rate of 100%. large family size(AOR=2.7, 95% CI: 1.06 – 6.9),children from families who had monthly income less than 1500 birr (AOR = 5.17, 95% CI: 1.7 -15.3), food insecure household (AOR = 2.9 ,95% CI:1.17-7.28) ) , not receive any nutrition information (AOR= 3.47,95% CI: 1.14 - 7.10) and lack of exclusive breast feeding (AOR = 2.69 ,95% CI : 1.18 - 6.10), no frequent hand washing (AOR= 7.6 95% CI:2.44- 23.6) as well as history of diarrhea two weeks prior to the survey (AOR 3.2 ,95%CI:1.4 -7.2) were the independent determinants of severe acute malnutrition. Conclusion: Family size, monthly income, food insecurity, lack of exclusive breast feeding, lack of information on child feeding, diarrhea and lack of maternal frequent hand washing practice were the determinants of severe acute malnutrition. Due emphasis should be given to strength awareness creation in the community towards family planning, exclusive breast feeding, frequent hand washing and prevention of diarrhea as well as improving household food security.