The practical importance of these studies comes from the fact that, the local degradation of dielectric layers due to the treeing discharge leads to the increment of energy losses and finally to the electric breakdown of the films. The thickness of the deposition layers was 1000 Angstrom. For the application of high voltage, we used the needle-plane system. The Mirror Electron Microscopy (MEM) investigations in polyethylene films without the application of high voltage have shown electrical inhomogeneities. After the application of high voltage (4 to 12 kV), thin polyethylene films have no electrical inhomogeneities because the sites corresponding to local conduction filaments are the starting points for pre-breakdown phenomenon.