This paper deals with common-mode (CM) electromagnetic interference (EMI) in an inverter-fed motor drive system. A simplified CM equivalent circuit including -type equivalent circuit for the motor winding is proposed by using the waveform of the common-mode current flowing in the ground conductor. Moreover, a parameter identification method applying the computer-aided software modeFRONTIER is proposed to decide objectively the circuit parameters of the proposed simplified equivalent circuit. Validity and effectiveness of the proposed equivalent circuit are confirmed by comparing the measured impedance characteristics with the calculated ones. Finally, the possibility of a nonlinear phenomenon is discussed in this system. It is shown that the accuracy of simulation result can be improved by introducing the nonlinear element. (c) 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.