Planar magnetics are widely used in high-frequency power electronics. Compared with wire-wound magnetics, they are with higher power density and less manufacture efforts, but higher loss density and operation temperature. It leads to the parameter shift, efficiency decrease, and even failure of the magnetics. This digest presents the degradation mechanism, failure criteria, and lifetime calculation of the planar transformers under thermal stress. The parameters and appearance change of the ferrite core, insulator, print circuit board (PCB), and core glue is analyzed. Three kinds of failure criteria are used to calculate the lifetime under different failure mechanisms. The study serves to better understand the thermal aging and reliability assessment of planar transformers for power electronics applications.