Project success or failure is strongly influenced by the level of plan and pre-project feasibility determination undertaken by key stakeholders. Most projects are unsuccessful to meet their objective because of in -depth assessment of the feasibility study and detail planning. Keeping this in mind, understanding the role of the feasibility study and detail planning within the whole project process is vital. The aim of this study is to investigate the role of detailed planning and feasibility study for project success: the case of water supply construction projects in north Gondar, Amhara region. The research method that was used in the study are observations and case study made in construction sites in addition to the interviews and questioner made with relevant personnel for water supply construction projects. Therefore the most important factor that necessitate to execute detail planning and feasibility study are mainly to make the project viable, for making a quality management system, cater foreseen time delays, to avoid cost overrun, and a sound financial turnover mechanism of the project. On the other hand, lack of using feasibility study and detail planning has an effect on the project success; therefore the result shows there are no satisfaction among the users, decrease project quality, increases time of the project duration/delays, increase cost overrun/cost under estimation, and improper project management. Furthermore, feasibility study and detail planning highly affect the project performance towards success. The study result also reveals that the performance indictor which is customer satisfaction has direct relation with feasibility study and detail planning, cost efficiency and effectiveness, production quality, construction timely performance as per the project schedule and employee’s satisfaction respectively. Finally, the framework of the feasibility study was presented and shows that the processes, tasks and tools that provide guidance and structure for the execution of a project since most of the water supply projects have no frame work regarding to feasibility study and detail planning for their work implementation. The conclusion indicates that detail planning and feasibility study have great role for both project performance and success. Key Words: Feasibility study, Detail Planning, Project performance, Project success