This work concerns the effect of thermal aging on breakdown voltage of polyesterimide enamelled copper wire. Samples are aged in ovens at different temperatures. After aging, a large population of samples were taken and submitted to dielectric breakdown under AC and DC voltages. The material was characterized by TGA and DSC. The values of failure were treated statistically using Weibull model. The study shows a decrease in breakdown voltage with respect to aging time. DC breakdown voltage is greater than AC breakdown voltage. Under DC stress, breakdown voltage is related to the polarity. TGA thermograms exhibit one decomposition stage. The beginning temperature of the mass loss and the residue are greater in the case of pure samples. The glass transition temperature decreases as a function of aging time resulting to a plasticization in the material. In long-term, the decomposition occurs by a scission reaction of imide - bond and ester – bond at interface of polyesterimide-copper. A through discuss of the obtained results is given as well.