The flyback transformer is a crucial part of the flyback converter, which has the highest portion of total power losses for most applications. Accurately prediction of the power losses due to flyback transformer early in the design stage provides an advantage about revealing reliable prototypes. In this paper, several loss prediction methods are applied for a 40W universal input and single output DCM flyback transformer as; analytical calculations via well-known Steinmetz and Dowell's formulas, a simulation including numerical solutions, and a finite element analysis (FEA) based investigation. It is shown that the comprehensiveness of the methods accounting for hysteresis losses, eddy current losses, DC bias, and fringing effect based losses determine the accuracy of the results. In the paper, it is shown that the more the comprehensiveness, the closer the results to the values of electrical and thermal measurements. The power loss results obtained from theory and measurements with five different methods are compared and discussed in the detail.