A new trap-assisted two-step tunneling model is proposed to explain the conduction enhancement characteristics and conduction mechanism in heavily-nitrided oxide films. A theoretical calculation is carried out to fit the theory to the experimental results. The trap density and trap energy level are found to be in the range of 1.2×1019-7.2×1020cm-3 and 2.46-2.56 eV respectively. These results agree satisfactorily with the Auger spectroscopic data. Furthermore, this model can also be applied MNOS structure or MIS devices with other traps.