The breakdown characteristics of very thin nitrided silicon oxide film have teen exammed. The breakdown mechanism of this film and the factors affecting the breakdown characteristics have been explored. It has been found that the improvement of breakdown properties of this nitrided oxide depended greatly on the changes of the microstructure of the film and interface, as well as the composition of the film. The narrowing of the band gap width of the film after nitridation makes the electric field for intrinsic breakdown decrease, but the occurrence of breadown may be greatly postponed by the dense material, the smoothed interface and the effect of traps. Experimental evidences confirmed that the im-pact-ionization exists in the intrinsic type of breakdown, but the permanent breakdown is caused by the heat transfer for all cases.