Abstract: Power bus noise problem and EM interference （EMI） problem caused by resonant impedances have become a major concem for EMC engineers engaged in PCB design. However, Electromagnetic Band-Gap （EBG） is an array structure periodically arranged by the dielectric, metal or combination of them. The use of the proper EBG structure can reduce not only the magnitude of the transfer impedances but also the number of the resonance peaks, thus contribute to the EMI reduction of the power bus. In this paper, the fast algorithm based on the cavity-mode model is combined with the segmentation method to calculate the impedance characteristic of a power bus whose dielectric layer is an EBG structure composed of two different dielectric materials. It is shown that EBG structure can significantly contribute to the EMI reduction.