Through analysis of thermally stimulated polarization and depolarization currents (TSPC and TSDC), dielectric properties of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are compared. Since the two polymers have very similar chemical structures, their dielectric properties are also similar. However, the temperature at which TSPC or TSDC starts to increase rapidly is about 20 °C higher in PEN than in PET. This difference is mostly due to the fact that the glass transition temperature Tg is higher in PEN. This means that thermal properties including the glass transition affect significantly the dielectric behavior of PET and PEN. At temperatures about 30 °C lower than Tg, the two polymers show a hump in their first-run TSPC spectra, probably originating from impurity or moisture. It is also clearly demonstrated that both PET and PEN can be reliable electrical insulating materials, especially at temperatures below their Tg's.