In Ethiopian, under a government regulatory body, i.e. Ethiopian Roads Authority, number ofrnroad construction projects is increasing from time to time. However, it becomes difficult torncomplete projects in the allocated budget. Cost overrun is one of the major problems of roadrnconstruction projects undertook by Ethiopian Construction Works Corporation, TransportrnInfrastructure Construction Sector. This research was carried out to assess and rank the severityrnof major factors that cause cost overrun during construction projects in the Corporation.rnQuestionnaire surveys together with desk study were used to collect data on time and costrnoverrun. A total of 82 responsescomprising 32 clients, 38 contractors and 12 consultants wererncollected and a desk study of 7 completed road construction projects in Ethiopia wererninvestigated. The data was subjected to descriptive statistical analysis and subsequently meanrnscores were determined to rank factors level of severity. From the study it was found that exceptrnone which was completed under estimated budget, the majority of road construction projectsrnfinished with severe cost overrun. The rate of cost overrun ranges from a minimum of 14.85 % tornthe maximum of 37.32% of the work executed and certified amount. The study identified 31rncauses of cost overrun for the case of Ethiopian Construction Works Corporation, TransportrnInfrastructure Construction Sector. The most important causes of cost overrun were found to bernineffective way of management of Material and Machinery resources at project (3.56), Skill ofrnSite Management Crew (3.53), wrong method of System/Approach employed to manage siternactivities (3.44), contract management (Inadequate knowledge and skills of Project Managementrnand Contract Administration teams) (3.43) and Design errors (3.4) are the five most importantrnfactors affecting cost of construction projects. It is likely that these findings will help focus onrnvery significant factors to improve the construction performance of the Corporation in the future.