The Si/SiO2 interface is attracting new interest as gate dielectrics in MOS devices become ultra thin. In this paper, the impact of pre-gate cleaning on the morphology of the Si/SiO2 interface and the electrical performance of CMOS gate oxides has been systematically investigated. Using the High-Resolution Transmission Electron Microscopy (HRTEM) technique, we observed the Si/SiO2 interface at an atomic level. We have found a direct experimental relationship between the pre-gate cleaning scheme, Si/SiO2 interface morphology, and the electrical properties of CMOS gate oxides. When the ratio of H2O2:NH4OH ≥ 1.45, the roughness of the Si/SiO2 interface was dramatically improved, which, in turn, increased the Charge-to-Breakdown to an ideal value.