The construction industry has a huge contribution to the overall economy of fast growingrncountries like Ethiopia, but cost overrun is causing a negative impact on it. To overcomernthis issue, the study aimed at identifying the causes of cost overrun in public buildingrnconstruction projects. A descriptive survey was conducted on 42 professionalrnrespondents who are actively involved in the construction industry. The samples wererndrawn using purposive sampling since the objective was to select participants who havernknowledge & experience in the issues being investigated. Primary data was collectedrnusing the questionnaires and secondary data is collected through reviewing of relatedrnliteratures. The study found that most important causes of cost overrun are poorrnpreplanning process accomplished by the contractor, inaccurate cost estimation by therncontractor, cash flow difficulties faced by the owner of the projects, economic instabilityrn& market inflation pressure on the price of resources, delay in decision making onrncritical issues by the consultant and poor financial management & control practicesrnundertaken by the contractor are the top major factor that causes cost overrun in publicrnbuilding construction projects. Moreover, the study found out the most effectivernmitigation measures for cost overrun verified by the experts are proper project planningrnand scheduling by the contractor, appropriate scope definition of the project by the clientrn&the consultant and realistic cost estimation by the contractor before contract signing.rnTherefore, it was concluded that even though the condition and the extent differ all keyrnstakeholders (client, consultant, contractor, and the external factors) have a significantrncontribution for the construction cost overrun. Finally, the study recommended that therntop management of ECWC should capacitate and provide more supervision to the teamsrnwho are responsible for the preparation of the cost estimation for the bid and properrnpreplanning process before kicking off the project execution.