Background
In Ethiopia about 75% of the landscape area is below 2000m above the sea level and affected by malaria cases. On June,18,2021, rumer of unexpected increase of malaria case in Harsu and Buno kebele was notified , the aim of investigation was to detrmine the cause of the outbreak and identify risk factor associated with malaria outbreak.

Methods
Unmatched case-control study conducted in 1:2 ratio, controls were from the same area with cases of those who have not developed symptom of malaria. The sample size was calculated by using Epi info version 7.2.1. , SPSS version 21 was used to analyze associated risk factors. Bivariate and multivariate analysis was done to determine the significance of risk factors of the outbreak.

Results
From the total of 66 cases 42 (63.6%) were positive for plasmodium. Significantly associated risk factors for malaria case were presence of stagnant water near to home or vicinity (AOR= 5.112, CI 1.865, 14.013), presence of intermittent river close to the community (AOR= 4.939, CI 2.116, 11.526), people who have gutter to collect rainy water and open deep well to store rainy water.

Conclusion
Plasmodium falciparum was the most dominant specious. There were mosquito breeding sites like river, stagnant water and open deep well which were responsible for an outbreak.