Electrical conduction in ultrathin nitrided oxide films on silicon has been investigated by comparing it with that in ultrathin oxide. Significant current enhancement in the nitrided oxide and partial recovery of the enhancement by O2annealing were observed, which can be satisfactorily explained by a two‐step tunneling via traps generated in the nitrided oxide during nitridation. From the comparison between theoretical calculations of tunneling current and experimental data for the nitrided oxide and the SiO2, it is found that the effective barrier height for electrons at an aluminum‐nitrided oxide interface decreases from 3.1 to 2.5 eV for an as‐grown nitrided oxide and to 2.75 eV for a nitrided oxide annealed in O2. The nitrogen‐related traps are estimated to be situated at the surface portion within 32–34 A˚ from the metal‐insulator interface with the energy level of 2.87 eV for the former and 3.32 eV for the latter below each conduction‐band edge.