In this paper, a new method is proposed for determining the parameters of Thevenin and Norton equivalent circuits. The method is unified and employs only one topology that depends on the test load impedance, Z(x), connected to the output of the circuit. It does not require setting all independent sources to zero for determining Thevenin and Norton impedance. All equivalent circuit parameters are derived simultaneously and systematically from the characteristic equation relating to the output of the circuit. (C) 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.