Stability and safety analysis of dams is a subject which has been studied in recent years by several researchers. Researchers have used various methods and models for studying the stability of the dam. Several methods are proposed for the analysis of earth dams under earthquakes. Some of these methods are viscoelastic methods and semi static methods. Each of these methods can be used as an appropriate method, but it can never be substitute of dam’s dynamical analysis. This study examines the effect of foundation on the dynamical behavior of earth dams. In this study, the foundation has been modeled by massless technique with simple near boundary conditions which has been used commonly in modeling of dam foundation. The effects of artificial earthquake spectrum with different range of frequencies are considered. The results show that the changes of foundation stiffness significantly affect the structural response of dam foundation and the foundation must be considered in analysis of earth dam.