The purpose of this study is to investigate the effects of electrical stress on the 1/f noise behavior in n-channel metal-oxide-semiconductor transistors with ultrathin gate oxides. Even under a weak electrical stress, the drain current noise (S-id) of the device with a 1.4-nm-thick oxide was found to increase abruptly beyond a certain critical gate bias. This deteriorated noise property was proven to be from simultaneous increases in gate current noise (S-ig) and the correlation between Sid and S-ig which were directly related to oxide trap generation and gate/drain current (I-g/(d)) ratio, respectively. Meanwhile, the increase in Sid in the device with a 2.3-nm-thick oxide after stress, with a comparable transconductance degradation, was relatively insignificant because of the device's smaller I-g/I-d ratio, even if the measured S-ig was comparable to that of the thinner oxide device. Consequently, the 1/f noise degradation could be much more significant than the accompanying DC characteristic degradations in the thin gate oxide below 1.5 nm.