Effect of endothermic reaction associated with glass transition on the dc and impulse breakdown strength of poly-L-lactic acid (PLLA) and polyethylene terephthalate succinate (PETS) was studied experimentally. Impulse breakdown strength decreases monotonically with an increase in temperature for both PLLA and PETS. However, it increases and shows a hump when the temperature approaches a certain temperature that is slightly lower than the glass transition temperature. Then it decreases again. Moreover, the degree of increment in impulse breakdown strength at this hump depends on the crystallinity of PLLA. In the case of dc breakdown, a similar local maximum as seen in impulse breakdown is not observed. Three possibilities, namely energy absorption and structural change both due to glass transition, and space charge formation, are discussed to explain the experimental results.