A Large scale hydropower dam known as Grand Ethiopian Renaissance Dam (GERD) is under construction on the Blue Nile River just upstream of the Ethiopian-Sudan border in Ethiopia. The GERD has an active storage capacity of more than 60,000 Mm³ and is anticipated to produce 6000 MW of hydropower energy. The aim of this study was to assess the potential impact of the dam on the performance of the High Aswan Dam (HAD) during filling and operation phases using a simulation model using Mike Basin river basin simulation model. The results indicate the planned 6 years filling period is sufficient to fill the reservoir with little impact on the current irrigation water demands from HAD in Egypt without additional management investment. There will be about 12% and 7% of reduction of annual energy output from High Aswan Dam during the filling and after filling stage of GERD respectively. Cumulative Energy production from the two dams will increase significantly Water loss at HAD due to evaporation will decreases by 22%. Overall performance of HAD during and after filling of the GERD remains at a reliability level of 96%. The study advises to utilize the results cautiously as it is based on only one historical realization of the many possible scenarios that may evolve in the future.