For isolated power converters, both conducted and radiated electromagnetic interference (EMI) are dependent on the common-mode (CM) performance of transformers. This paper proposes an RLC ladder network modeling and its parameter extraction technique to characterize the wideband CM properties of transformers at the circuit level. It is proved that the ladder network covers detailed electric and magnetic parameters, therefore, is more applicable for wideband than commonly used lumped models. The finite element method (FEM) is selected to extract parameters for accuracy. Influential factors on parameters, including geometries and material properties, are also presented. With S21 parameter as the evaluation method of transformers’ CM performance, the proposed modeling technique is verified from 100 kHz to 200 MHz on a planar transformer through the comparison between measured and simulated S21 parameter.