In order to fabricate metal‐insulator‐semiconductor (MIS) devices with gate insulating films thinner than 5.0 nm, organic monolayers have been grafted on the native oxide layer of silicon wafers. We demonstrate that a single monolayer of octadecyltrichlorosilane with a 2.8 nm thickness allows to fabricate a silicon based MIS device with gate current density as low as 10⁻⁸ A/cm² at 5.8 MV/cm, insulator charge density lower than 10¹⁰ cm⁻², fast interface state density of the order of 10¹¹ cm⁻² eV⁻¹, and dielectric breakdown field as high as 12 MV/cm. Moreover, this insulating film is thermally stable up to 450 °C.