To provide basic considerations for the realization of methods for predicting the electromagnetic (EM) radiation from interconnected printed circuit boards (PCBs) by a coaxial cable, the characteristics of the EM interference of test model PCB are investigated in this paper by experiment and numerical modeling. Comparing the cases with and without the interconnection, the ground plane of the interconnected PCBs and the interconnection cable are dominant radiation factor in the lower frequencies. These factors construct large common-mode (CM) antenna. On the other hand, the total EM radiation at GHz frequencies is simply determined by the differential-mode (DM) current on the signal trace, and cross-talk current on the victim traces. Consequently, correct predictions of CM current at lower frequencies and cross-talk are keys to the prediction of the total EMI behavior of the interconnected PCBs, because the DM current on the signal trace can be predicted by transmission line theory.