Investigation of the behavior of dams against seismic loads is a key factor for dam safety requirements. One of the most important problems in evaluation of seismic behavior of concrete gravity dams is dam-reservoir-foundation interaction. Hydrodynamic pressures generated due to seismic forces and Fluid-Structure-Soil Interaction (FSS); are inevitable. In this paper, the basic equation involved the water-structure-foundation interaction and the effective factors are considered for concrete gravity dams. Dam-reservoir-foundation interaction has been investigated utilizing seismic analysis. 2-D damreservoir-foundation coupled system is analyzed using FEM via ANSYS code. Dam and foundation are assumed to be linear and elastic while reservoir water is considered acoustic, inviscid and incompressible. The dam and foundation have been idealized by considering linear, elastic and plane stress conditions. The modeling of reservoir has been carried out by fluid acoustic element and proper consideration of fluid boundary and initial conditions. The effect of foundation flexibility has been obtained by considering various dam-foundation rock interaction ratios i.e. modulus of elasticity of foundation to modulus of elasticity of dam. Results show that both foundation mass and flexibility have an outstanding impact on the behavior of dams and is necessary to consider their impact while simulating seismic response of concrete gravity dams.