The properties of geomaterials are uncertain. These uncertainties not only affect the dynamic behavior of the geomaterials, but also significantly influence the complex nonlinear dynamics between the soil, foundation, and the structure (dynamic soilfoundation- structure interaction - SFSI). However, current civil engineering approach in incorporating the effects of SFSI on the seismic behavior of structures is still largely deterministic without considering uncertain geomaterial properties. The objectives of this research are to characterize the uncertainties in soil properties m a probabilistic framework and to quantify their effects on dynamic soil-foundation system behavior during seismic loading. The research methodology includes systematic propagation of uncertainties in soil properties through soil-foundation interface to the dynamic behavior of the structure during seismic loading. A recently developed Contact Interface Model (CIM), to model the soil-foundation system behavior during seismic loading, has been used in numerical simulations. To study the sensitivity of the response of the soil-foundation system to the random input parameters, probabilistic analyses have been carried out using Tornado Diagram analysis, Spider Plot analysis, First Order Second Moment (FOSM) analysis, and small scale Monte-Carlo simulations. Results obtained from the probabilistic numerical simulations indicate that ultimate moment capacity of the soil-foundation system during seismic loading is more sensitive to the uncertainty in the applied vertical load on the foundation than the uncertainties in soil properties. Since the uncertainty in applied vertical load is considerably smaller than the uncertainties in soil properties, the ultimate moment capacity of shallow foundation is predictable with reasonable accuracy. Energy dissipation beneath the foundation mainly depends on the applied vertical load and initial vertical stiffness of the foundation, while initial vertical stiffness of the foundation and rebounding ratio were found to contribute the most to the settlement of the foundation. The rotation of the foundation is more sensitive to the shaking intensity than uncertainties in soil properties.