In the traditional systems of irrigation and exploitation of water sources, especially the water sources behind dams, there isnt regular and consistent plan for their management and this can be caused the weakness of irrigation performance. In this connection, using both of optimization methods and Geospatioal Information System (GIS) together can improve the performance of irrigation projects. On this basis, this paper uses Linear Programming (as an optimization method) and GIS in downstream lands of Aq-chay dam to allocate optimal area for cultivation of any agricultural product that will be caused the improvement in the performance of irrigation projects. In this regard, a program in Microsoft Excel was developed based on Linear Programming with respect to type of products, decision variables, constraints and objective function. Then according to its output, another application was developed using Arcobjects capabilities in ArcGIS software and hereby the optimal areas for the cultivation of any agricultural product in downstream lands of Aq-chay dam was determined. The result indicates that the efficiency has been improved up to 30 percent in arable surface and type of products in the region of case study, using the Linear Programming and GIS in comparison with traditional systems.