This paper shows that there is an intimate relation between the TG data and long term thermal endurance of dielectrics, TG curves are able to reveal the thermochemical stability of the dielectrics and are main factors governing their thermal capability. But, they are in principle different from the thermal endurance properties of insulating materials again. This is chiefly because TG data are lacking in the functionality, that is, can not reflect the physical thermostability of materials, further, both reaction mechanisms have certain differences as well. The paper still gives the comparison between rapid thermal endurance test methods and conventional aging method, and analyzes why errors of Cut line method are excessive.