Background: key essential medicines are those pharmaceuticals which must be available in sufficientrnquantities at all times for the management of most common disease. However, different studies shows thatrnIn Ethiopian public health facilities, stock outs of key Essential medicines is common. Hence this study wasrnaimed at assesing key essential medicines Inventory management performance at public health facilitiesrnfound in south west Shewa zone of Oromia region, Ethiopia. rnMethods: Mixed method design were employed from June 1/2021 to June 30/2021. The quantitative datarnwere collected through semi structured questionnaires, and observational checklists. The qualitative datarnwere gathered through key informant interviews with the responsible professionals. The quantitative datarnfrom the questionnaire were analyzed using the Excel spreadsheet version 16 to calculate, stock out rates,rninventory accuracy rate, percentage of acceptable storage condition, percentage of facility submitted reportrnon time and percentage of facility that receive exact quantity drug ordered. Chi square test was conductedrnto determine significant variables. Whereas the qualitative part of the study was transcribed, summarizedrnmanually and analyzed using thematic analysis. rnResults: Inventory accuracy rate for key essential drugs was found to be 76% and 72.5% in hospitals andrnhealth centers respectively. Out of twenty five key essential medicines assessed, the overall mean stock outrnwas 24.99%. On average, 93.3% of bin card were updated. Only three health facilities under study met thernacceptable storage criteria and the reporting rate of the facilities was determined to be 88.8 %. The causesrnof stock out were shortage of supply at Ethiopian pharmaceutical supply agency and in the market, lack ofrntrained human resource, lack of communication and budget constaints. rnConclusion and Recommendations: Over all, Inventory management performances of key essential rnmedicines needs improvement. and shortage of supply at EPSA and on the market, inappropriate storage rnand reporting practice and lack of automation were the reasons for stock out within review period. Thus,rnhealth facilities should work in harmony with stalkholder to ensure uninterrupted supply of Key essentialrnmedicines .