A new modified model for assessing the effects of SSI on base-isolated structures is presented from the case of conventional structures. The equations of motion of the equivalent fixed-base 2-DOF system are formulated in frequency domain. Based on this, rigorous and closed-form formulations for fundamental period and composite damping are derived. Finally, analysis of parameters based on frequency independent coefficients in detail is conducted. The analytical model and method presented can serve to assess the effects of SSI in the design of base-isolated structures including high-rise structures. Significant differences between the system with and without radiation damping and the flexibility of supporting soil have been observed in this study. For the results of the analysis, overall SSI effects have been identified as elongation of fundamental period of the system and decreased composite damping, especially for the high-rise and slender isolated structures.