Medium frequency transformer (MFT) is considered to be the key element in a solid state transformer (SST). Compared to the conventional line-frequency transformer, the MFT has much reduced volume and weight. However, the MFT suffers from higher core loss due to its medium operating frequency. Normally, the MFT core loss is experimentally measured by open-circuit test, when the magnetizing voltage and magnetizing current are measurable. Whenever the MFT is not open-circuited, the direct in-circuit core loss measurement is impractical. In this paper, a simple technique is proposed to achieve the in-circuit core loss measurement for the MFT under arbitrary excitation voltage waveforms and various load conditions. The operating principle and key steps of the proposed measurement are explained in details. The accuracy of the proposed measurement technique is verified by a resistive load experimental test under both sinusoidal and non-sinusoidal excitation voltages.