This paper presents the result of modeling the magnetic characteristics of ring-shaped cores made of Fe Co Nb Cu B alloy in both as-quenched and nanocrystalline state. For the modeling, the extended Jiles–Atherton model was used. Extension of Jiles–Atherton model is focused on taking into account the changes of parameter caused by changes of the average energy required to break pinning site. Determination of model's parameters was made on the base of optimization process. This optimization covered both evolutionary strategies and simulated annealing. Finally, good agreement between experimental characteristics and the results of modeling was achieved. This agreement is confirmed by the high value of determination coefficient, which exceeds 99% for both as-quenched and annealed cores. As a result, it was confirmed that the presented method of modeling the magnetic characteristics of FeCoNbCuB alloy in both as-quenched and nanocrystalline state is suitable for technical applications.