We study ester-type liquid crystal, 4'-cyanophenyl 4-n-hexyl benzoate and its homologue 4'-cyanophenyl 4-n-pentyl benzoate. We used the Thermally Stimulated Depolarization Currents method and found information about phase transition temperature, and activation energies. The temperatures of phase transitions were compared to those obtained by classical DSC/DTA measurements. Simultaneously with the TSDC, we measured the optical transmission; we found a decrease of the optical transmission when it was measured after a greater pre-applied polarizing electric field.