This time the Ethiopia government gives its attention in the provision of infrastructure as it is the basic for the development of one country. In reflection of this consent Design and Construction of new Water supply Projects are widely seen. So many private (domestic and foreign) and governmental organization are participating on the sector in the study, design, construction and supervision field.There is an increase in the number of construction projects experiencing extensive delays leading to exceeding the initial time and cost budget. The main objective of this research were to assess Causes and Effects of delay on ‘Legedadi Deep Wells Water Supply Design and Construction Project (Phase-I). As per the original signed contract agreement, the project is supposed to be completed in 12 months with the total allocated budget of ETB 1.35 Billion while due to time overrun the project have been completed in 15 months and consumed at the end of the project ETB 1.41 Billion which is higher by ETB 0.06 Billion or 4.44% than the contract amount which creates challenges to allocate additional budget on the client side, inefficient service delivery on the project owner, Addis Ababa Water and Sewerage Authority. The study adopts quantitative and qualitative methods with the help of primaryand secondary data. Primary data was collected using self-administered questionnaires on 54 respondents and key informant interviews with 15well experienced in such specific water supply construction projects selected from the project owner, consultant, contractor and subcontractors. Secondary data was collected through reviewing of project documents.Such as; BOQ, Contract agreement, design documents, formal communication letters, and change orders etc. vi Respondents were asked to rank the importance general and specific factors of delay causes using five points scale (1=Very low, 2= Low, 3= Average, 4= High and 5= Very High)and the analysis shows that, top causes of project delay were related to the Client, Consultant, Contractor and sub-contractors. Major findings of this research are: ✓ The construction work was started based on design prepared on inaccurate inputs and assumptions, insufficient field investigation due to unrealistic three months deign period. ✓ ETB 60,000,000 additional cost (Variation) incurred and the Employers plan to collect ETB 20,007,000 from water selling in three month time has not been achieved. ✓ Due to incorrect Engineering cost estimation excess Pipes and materials purchased from abroad and ETB 81,846,397.81 Employers capital is unnecessarily tied up because total design length has been 107.52km and after project completion the actual total installed length becomes 89.48km which is less by 18.05km. ✓ Promise has been announced repeatedly to the public through mass media about the delivery day of potable water to the residents of condominium houses in the expansion area. However, the project has been completed and put into service after Three month delay and this creates public grievance due to low performance of the project owner. The study suggested that current project planning, monitoring and evaluation system, starting from study/design stage up to commissioning date have to be improved by adopting the right tools and tested procedures, assigning the appropriate staffs from both vii the project owner and the consultant, establish accountability and by applying timely knowledge based decisions so that, causes and effects of delay can be managed. At large, research needs to continue in order to refine and find more effective mechanism to minimize if not avoid, causes and effects of project delays which are currently a chronic challenge of our country, Ethiopia.