In this paper, dipolar relaxation processes in epoxy-based polymer were investigated under various polarization conditions, using thermally stimulated depolarization current (TSDC) technique. The TSDC spectrum of un-poled epoxy polymer reveals an unusual negative current, associated with a thermally generated charge carriers, which were due to impurities ionization and structural changes mainly at high temperature (above Tg). β and α dipolar relaxations were detected respectively around 115 °C and 154 °C in conventionally poled sample at TP=110 °C and EP= 3 kV/mm. Dipolar relaxation parameters were evaluated using two techniques: (1) theoretical decomposition of global complex TSDC spectrum, using Bucci-Fieschi expression based on single Debye process, (2) experimental procedure based on selective polarization, commonly known as windowing polarization (WP). The obtained values are TP dependent: relaxation time decreases and activation energy increases when polarization temperature increases. An ω relaxation peak due to water molecule departure was detected around 135 °C, using WP techniques for TP=100 °C Unlike dipolar relaxations, its position is independent of polarization temperature. Complementary thermal analysis investigations by differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA), were made to support some given conclusions.