In this paper, the impact of Si surface roughness on metal oxide semiconductor field effect transistor (MOSFET) characteristics with ultrathin hafnium oxynitride (HfON) gate insulator formed by electron cyclotron resonance (ECR) sputtering was investigated. The surface roughness of Si substrate was reduced by Ar/4.9%H₂ annealing utilizing conventional rapid thermal annealing (RTA) system. The obtained root-mean-square (RMS) roughness was 0.08nm (as-cleaned: 0.21nm). The HfON was formed by 2nm-thick HfN deposition followed by the Ar/O₂ plasma oxidation. The electrical properties of HfON gate insulator were improved by the reduction of Si surface roughness. It was found that the current drivability of fabricated nMOSFETs was remarkably increased by reduction of Si surface roughness.