Considering the flexibility of the foundation leads to soil-structure interaction (SSI) analysis. Because of the dependency of soil stiffness and damping on excitation frequency, in the conventional approach, both soil stiffness and damping are refered to as the complex stiffness or the so-called impedance function. But in case of using the impedance function, it is impossible to perform a nonlinear soil structure interaction analysis. However, most of base isolation systems enter the inelastic region very quickly, while excited by an earthquake. Therefore in such cases, an alternative approach to describe soil stiffness and damping must be used. On of these approaches is the lumped parameter method. To study the effect of soil-structure interaction on isolated structures, firstly the relevant parameters on soil structure interaction and dynamic response of isolated structures must be spotted. Afterwards the effectiveness of each of these parameters should be accounted for. The most effective feature of foundation on SSI is the shear wave velocity. Concerning the friction pendulum systems (FPS), which is the focus of this study, the mass of structure and the bearing radius, are the most important factors. It turns out that the ratio of the isolated structure period to the initial structure period is also of some importance. Paying special attention to these parameters, this thesis study the effect of SSI on friction pendulum base isolated structures. An isolated structure is modeled by SAP2000 software for tow cases of rigid and flexible foundation. After a comparison between the results, the shear wave velocity and the isolation period seem to stand out as the most effective factors on the dynamic response of friction pendulum base isolated structures so that if the product of the isolation period and the shear wave velocity falls within a certain interval, considering SSI have a detrimental effect on seismic response.