The high field electric breakdown of polyethylene/montmorillonite nanocomposites was studied in detail, and compared to the unfilled respective films. The electric breakdown strength (EBD) of ‘aligned’ composite films (i.e., films with fillers oriented parallel to the film surface) is much higher than the EBD of the respective ‘isotropic’ composites (i.e., films with fillers of random orientation) and of the unfilled polymer films. This behavior suggests a barrier mechanism as the origin of electric strength improvement, a supposition that was investigated in detail here and is supported by: (a) EBD is not related to filler-induced or strain-induced polymer crystallinity changes; (b) EBD decreases with modulus, but improves with toughness for these films; (c) there is no change in the polymer EBD for a small temperature jump (ΔT from 25 °C to 70 °C), but there is a definitive change in the composite EBD for the same ΔT. All these, support the postulation that the electrical breakdown of these systems is predominately through thermal degradation mechanisms, which can be affected by the existence of oriented inorganic nanofillers (and of oriented polymer crystallites). Thus, the controlled orientation of nanofillers is shown to be an effective approach to substantially improve the electric breakdown strength of PE dielectric/insulating films.