This paper presents the modified core loss calculation of the high-speed (HS) permanent magnet synchronous machines (PMSMs) with amorphous metal (AM) stator cores. In the initial stage of the HS PMSM design, the calculation of stator core loss is obtained according to the data of AM ribbon provided by the manufacturer, which causes a large error of AM core loss. So how to make the accurate calculation of the AM core loss with only the AM ribbon data from the manufacturer is impending in the initial and advanced design stages of HS PMSM development. In this paper, the characteristics of AM ribbon and AM stator core is measured and compared. According to the experimental results, the modified factor for the accurate calculation of AM core loss is obtained based on the loss characteristics of AM ribbon. Then, the no-load loss of a HS PMSM prototype with an AM stator core is analyzed by numerical methods and verified by experiments. The proposed modified factor of the AM core loss calculation can provide a reference for the design and analysis of HS AM PMSMs in the industry.