Abstract: To accurately calculate the core loss of transformers over broadband frequency range, the hysteresis properties and loss properties of ferromagnetic materials were measured at different frequencies and analyzed based on the loss separation model. Different forms of real and complex permeability were used to express the constitutive law between magnetic induction and magnetic field strength in order to consider the influence of skin effect on eddy current loss calculation. It was found that the permeability should be regarded as a variable with frequency instead of being extracted from the approximate DC magnetization curve when it was used to approximate the constitutive law. Based on the above conclusion, the magnetic field in ferromagnetic materials was separated based on the loss separation theory and an approximate permeability was proposed based on the field corresponding to the anomalous loss so as to predict the loss characteristics of material in a wide frequency range based on less data measured at low frequencies.