The Ethiopian Government began constructing The Grand Ethiopian Renaissance Dam (GERD). This dam is expected to affect Sudan which is dependent on the Blue Nile. In this situation there is a clear need to assess these downstream impacts. Against this background, a simulation and optimization model for a multi-purpose cascade of reservoirs has been developed to provide an optimal operation of the Blue Nile System in Sudan for both current and future situation (before and after GERD becomes operational) under three scenarios (dry, average and wet years). Two objectives were considered in the development of this optimal operation: maximizing hydropower generation and maximizing the new irrigation area that becomes potentially available after the heightening of Roseires. The model was called the Excel-based Blue Nile Simulation and Optimization Model or EBSOM. The results of EBSOM before GERD becomes operational showed that the maximum area that could be planted was 525,000 hectares. This area will decrease the annual energy production of Roseires reservoir by 9.1% at minimum and 23.5% at maximum. The results showed that if the filling and operation of GERD is carried out in consultation with Sudan and Egypt, it can be expected that there will be several benefits especially in terms of irrigation and hydropower generation. However, there are also some negative consequences that need to be further studied and addressed.