The Jiles-Atherton (J-A) model is widely used in the modeling of the magnetization process of magnetic cores, but its simulation accuracy in the nonlinear section of hysteresis loops is low due to its theoretical defect. To improve its simulation accuracy, a method for reducing simulation errors arising from the original J-A model is proposed, and thereby a modified J-A model is established. Measured hysteresis loops of nanocrystalline alloy cores at different frequencies are simulated using the original J-A model, and simulation errors are concentrated mainly in the nonlinear section of the hysteresis loops. Simulation results of the original J-A model are modified using the modified J-A model, and thereby the simulation errors in the nonlinear section are greatly reduced. This indicates that the modified J-A model can more accurately describe the magnetization process of cores.