In recent years, effective methods for predicting and suppressing EMI over a broad band are required. In this paper, we newly focus on prediction of EM radiation from a strip line structure by an equivalent circuit model. Firstly, frequency responses of common-mode (CM) current on the PCB and EM radiation are studied with FDTD modeling. It was demonstrated that the ground plane of the PCB is the dominant radiation factor at low frequencies. Secondly, an equivalent circuit model for predicting CM current was proposed. The equivalent circuit model is based on consideration of concepts of CM antenna impedance to current- and voltage-driven mechanisms. The good agreement between the predicted and full-wave results indicates the validity of the equivalent circuit model. The frequency response of EM radiation from microstrip/strip line structure can be identified using our proposed model.