In the electric circuit simulation the Spice is mainly used, and in the power-electronics circuit simulation it is not exception. In the power-electronics circuit simulation, the details of the nonlinear device models is important. Although, the reactance model which is one of the nonlinear devices is not enough to use with power-electronics circuit simulation. Therefore we proposed a reactance model equation which is based on the differential permeability. In this paper, the model equation of the differential permeability is described with a exponential function, and we confirmed experimentally that it equation approximate to B-H curve of a Mn-Zn ferrite core.