A new model used to interpret the phenomena of dielectric breakdown is proposed over the whole range of applied field. There are two main mechanisms that one of which is the avalanche breakdown, so-call intrinsic type, and the other is the filament heating transport leading to the destructive breakdown exist in the breakdown process. The theoretical analysis and the experimental data indicate that the field strength of intrinsic breakdown depends on the band gap and the thickness of film, but the filament heating transport which induces a destructive breakdown depends on the properties of film. In particular, the defect is the main origin leading to the occurrence of destructive breakdown. In addition, the effect of various physical parameters, testing conditions and process factors on the breakdown are discussed briefly.