Time‐dependent dielectric breakdown lifetime of 3.0–3.5‐nm‐thick silicon oxide films was investigated using metal–oxide semiconductors (MOSs) produced by our recently proposed continuous ultradry process. This process prepares an ideal MOS gate oxide with few hydrogen‐related defects and precise thickness distribution that drastically affect the lifetime of such ultimate ultrathin oxide films. The lifetime of 3.0‐nm‐thick oxide films was almost equivalent to that of 3.5‐nm‐thick films, although their lifetime was apparently different from the lifetime of 5.0‐nm‐thick films. Therefore, the stress‐induced charge trapping mechanism in such ultimate ultrathin films that mainly determines the lifetime is probably identifiable. This lifetime saturation also suggests that their lifetime may be close to the intrinsic maximum limitation for ultrathin silicon oxide films.