Nowadays, it is difficult to accurately measure the core losses of magnetic components of power converters under high-frequency excitation. Based on the DC power method, this paper proposes a measuring system for core losses at 10 MHz using a new wide-bandgap semiconductor device, GaN. The switching process of the DC/AC inverter circuit is analyzed in detail, and the loss model of the DC/AC inverter circuit is established. The measuring accuracy is verified with the air-core inductor, whose loss can be accurately measured, and the maximum relative error is 9.1%. Finally, the core losses of magnetic components under 10 MHz excitation are measured based on the built measurement system.