The carrier effective mobility &mgr;effin the inversion layer for bothn‐ andp‐channel metal‐oxide‐semiconductor field‐effect transistors with ultrathin gate dielectrics prepared by rapid thermal reoxidation (RTO) of rapid thermal nitrided (RTN) SiO2has been studied. It is found that although RTN/RTO degraded the low‐field &mgr;eff, it improved significantly the electron &mgr;effunder high normal field compared to control SiO2. The effect of RTN/RTO on the hole effective mobility has also been examined and found to be quite different than on the electron effective mobility. A physical mechanism is discussed to account for the observation.