In this paper, the effects of internal ionization process, which occur in the voids under strong electrical field, on the polymer dielectrics degradation with time are analyzed. Theoretical equations have been derived for partial discharge V-I characteristic, voltage and discharge current variation with time in the discharge channel, discharge channel cross-section area variation with time and voltage, and channel expansion. The results have shown that approximately 21% of released energy is used for channel expansion and similar to3% of released energy is used for atom ionization when resonant emission and heat transfer are ignored. Effect of discharge impulses on the surface of dielectric for different pi values has been compared. (C) 2003 Elsevier B.V. All rights reserved.