Electromagnetic interference (EMI) forecast is a critical issue in the modern electronic systems. The conventional method of EMI forecast includes the resonance frequency and S parameter of single board itself. However, it was not enough to estimate the worst peak frequency of EMI accurately, because on-chip capacitance affects greatly on the resonance frequency. Therefore, the EMI forecast is required to be improved by including on-chip power distribution network (PDN) and package PDN too. In this paper, transfer impedance of the entire system was precisely analyzed. As a result, it was confirmed that the peak frequency of transfer impedance of total system showed better correlation with the measured peak frequency of EMI.