Abstract: Frequency selective surfaces (FSS) are often loaded over antennas to enhance radiation performance or introduce specific functions. Because of the vast number of FSS unit cells with sub-wavelength size and the large electrical size of an antenna or antenna array, the full-wave simulations for an FSS loaded antenna consume many computational resources, especially for FSSs with complicated physical structures. To solve this problem, a method based on the generalized sheet transition condition (GSTC) is proposed for the fast analysis of antennas loaded with multilayered FSS. The surface susceptibility of the FSS is extracted using the GSTC, and then the physical structure of the FSS is replaced with a three-layer cascaded impedance surface (CIS). The three-layer CIS possesses the same scattering characteristics as that of the FSS. The impedance of each surface is expressed by the extracted surface susceptibility. In this method, massive meshes for the FSS analysis are not necessary, thus significantly improving the efficiency and alleviating the CPU time and memory requirements. The result of the proposed method agrees well with the full-wave simulation using HFSS and the CPU time and memory are reduced by about 90%.