In this study, Si(100) surface flattening process was investigated utilizing sacrificial oxidation method to improve Metal--Insulator--Semiconductor (MIS) diode characteristics. By etching of the 100,nm-thick sacrificial oxide formed by thermal oxidation at 1100$^{circ}$C, the surface roughness of Si substrate was reduced. The obtained Root-Mean-Square (RMS) roughness was decreased from 0.15,nm (as-cleaned) to 0.07,nm in the case of sacrificial oxide formed by wet oxidation, while it was 0.10,nm in the case of dry oxidation. Furthermore, time-dependent dielectric breakdown (TDDB) characteristic of Al/SiO$_{2}$(10,nm)/p-Si(100) MIS diode structures was found to be improved by the reduction of Si surface RMS roughness.