Introduction: Measles is a leading cause of childhood morbidity and mortality worldwide. Anrnestimated 164,000 people died globally from measles in 2008 mostly children under the age ofrnfive.rnIn Ethiopia, the expected case-fatality rate is between 3% and 6%; the highest case-fatality raternoccurs in infants 6 to 11 months of age, with malnourished infants at greatest risk.rnThe main aim of this outbreak investigation was to assess the magnitude and risk factor ofrnmeasles infection in Wadera Woreda of Guji zone, Oromia Region, from 04-19/12/2016.rnMethod: Descriptive cross-sectional followed by case-control study design was used to identifyrnthe risk factors. Unmatched case-control sample size determination with a ratio of 1:2 for casesrnand controls.rnResult: A total of 393 cases were reported. Six samples were taken and four of them werernpositive for measles. The Index case was female aged 20 years who was not vaccinated. Werndetected a period prevalence of 5.93/1000. In the variable analysis having previous measlesrninfection and being vaccinated with measles vaccine has a protective factor withrnAOR=0.36,95%CI:0.16-0.76,P.value 0.008 and AOR;0.41,95%CI:0.17-0.97,P.value 0.043rnrespectively. while being under five years old and having contact history with measles cases arerna risk factor for developing measles infection with AOR;2.83,95%CI(1.25-6.43),P. value 0.013rnand AOR;6.75,95%CI(2.69-16.94),p. value < 0.0001 respectively.rnDiscussion: Periodic measles outbreaks occur when a large number of susceptible accumulate inrna community. Susceptible accumulate through time even in the setting of high routine measlesrnvaccination coverage.rnConclusion: The main factors contributing to this outbreak was the accumulation of largernnumbers of susceptible children over time in the Woreda..rnRecommendation: Ministry of health, Regional health Bureau and Zonal health department tornuse the second opportunity of Supplementary Immunization Activities and have conducted arncatch up and several follow up campaigns to increase measles immunity by keeping data recordsrnat all levels.