With attention to total energy loss of injected electrons through Fowler–Nordheim (F–N) tunneling, time‐dependent dielectric breakdown (TDDB) of 5‐nm‐thick silicon oxide films was investigated. Metal‐oxide‐semiconductor (MOS) diodes with four combinations of gate electrode and substrate type were fabricated. This produced different energy losses of injected electrons at a constant F–N tunneling current stress in spite of the same oxide thickness. TDDB lifetime was strongly affected by this energy loss difference. Since other electrical changes caused by trapping of stress‐induced positive charges in oxide also exhibited the similar energy loss dependence to the lifetime, TDDB for ultrathin oxide films is probably dominated by the trapping of positive charges, which are mainly generated near anode‐side oxide interfaces through the surface plasmon mechanism.