The effects of the structure’s surrounding soil layer on the seismic response of the structure are investigated for decades. Most of the researches related to the soil-structure interaction (SSI) have been conducted with linear or at most equivalent linear assumption of the soil layer behavior. Considering nonlinear soil behavior and its effects on the seismic response of the total system is almost a new researching field relating to SSI and is known as nonlinear soil-structure interaction. The nonlinear soil behavior influences on both the free-field response and the soil-structure interaction in a complex way. In this study employing a new plasticity model developed for simulation of monotonic and cyclic behavior of sandy soils, the effect of nonlinear soil behavior on the elastic and inelastic response of a set of 5, 10 and 20-story R/C generic frame model is considered and compared with linear behavior of the soil layer. The modal pushover analysis (MPA) method was used to consider inelastic behavior of structures and nonlinear time history method to analyze soil-structure system which after conducting MPA analysis is a one degree freedom structure reflecting modal characteristic of the mulistory structure supporting on the flexible foundation. MPA method is generally recognized as a new analysis method for estimating fixed-supported structure demand with capability to include higher mode effects on response. Many prior researches were conducted to consider possibility and ability of MPA method in SSI problems. By using MPA approach to analyze structures supporting on flexible foundation, the effect of the foundation flexibility on the load distribution which is used to push system in each mode will be involved. The constitutive model used for considering nonlinear soil behavior was presented by Dafalias and Manzari (2004). They proposed the model as a bounding surface model based on the state parameter of soil. Formulation of the model is based on stress ratio and states that just change in value of stress ratio result in deviatoric plastic deformation. The volumetric plastic deformation is related to the deviatoric plastic strain by dilation parameter defined using Rowe’s dilatancy theory. Using plasticity theory consistent with the critical state framework and by usage of state parameter, this model provides a nonlinear constitutive model which could predict nonlinear monotonic and cyclic response of soil almost accurately. The nonlinear behavior of the structures was modeled using moment-curvature relations for beam elements and fiber-element method for columns. Fiber-elements could consider inelastic behavior of a member using multi-axial spring following nonlinear stress-strain relation of materials assigned to the section of the member. By using this method moment-axial load interaction of the columns will include automatically. To consider soil stratum stiffness effects on the seismic response of frames, three types of the soil layer with 150, 250 and 500 m/s shear wave velocity of soil halfspace were selected. The analysis was conducted for five near fault ground motions. Results are presented as response spectra for three types of generic frames with both elastic and inelastic behavior as well as linear and nonlinear soil behavior and also all type of soil layer. Foundation rocking and its effects on the seismic response of the structures was another aspect of this study. Keywords: nonlinear soil-structure interaction, bounding surface soil model, modal pushover analysis, response spectra.