An in-situ high-vacuum process has been developed for the preparation of ultrathin Al2O3 films on monolayer Se passivated Si(100) surface. The in-situ high-vacuum process prevents the Si surface from adsorbed moisture or native oxide, leading to an Al2O3/Si(100) interface without any low-k interfacial layer at 400°C. The Se monolayer terminates the dangling bonds on Si(100) before the formation of the high-k dielectric. The electrical properties of the Al2O3 film on Se passivated Si(100) are significantly improved in comparison to control samples without Se passivation. The interface trap density is about 5 times lower to 2×1011 eV-1/cm2. The C-V hysteresis is reduced to 80 mV from 160 mV. The leakage current for the same physical dielectric thickness is reduced by more than an order of magnitude. copyright The Electrochemical Society.