Polyimides (PIs) are widely used in electronics wherein high temperature (HT) insulating layers are needed. Like other polymers used in HT and in ambient atmosphere, polyimides are subject to the thermal and thermo-oxidative degradations. The thermo-oxidative aging can lead to the material dielectric properties degradation and then to the device failure. Therefore, it is important to study the thermo-oxidative aging effect on the dielectric properties of PI thin films, in order to predict their lifetime. During the thermo-oxidative aging of PI films deposited on stainless steel substrate at 300 °C in air, electrical (dielectric loss, DC conductivity, breakdown voltage), physical (thickness and surface profile) and physicochemical (FTIR) properties, have been monitored for different film thicknesses. Results are analyzed and discussed in order to choose the suitable life end point criterion, and to estimate the mean time to reach it in such aging conditions.