Massive structures such as concrete gravity dams are very complex with respect to safety and stability. The behavior of these dams under seismic loads plays an important rule for safety requirements. One of the most important problems in the evaluation of seismic behavior of such dams is the dam-reservoir-foundation interaction. Hydrodynamic pressures induced due to seismic loads and fluid-structure interaction effects must be included in the analysis. In this paper, the effect of dam-reservoir-foundation interaction on the seismic response of concrete gravity dams is presented. A 2-D Finite Element model is used to model the Koyna dam using ANSYS computer code. Both mass and flexibility of foundation are the main effective parameters considered in the present study. The foundation flexibility is expressed as foundation to dam module of elasticity ( for values of 0.5, 1, 1.5, 2, 5 and 10. Modal and time history seismic analyses are considered in the present study. Results of the study confirm that the foundation flexibility and mass significantly affect the natural frequency and crest displacement.