Silicon‐surface microroughness was formed by cleaning cycles of an NH4OH‐H2O2‐H2O solution. Not only the roughness of the silicon surface, but also the roughness of the thermally oxidized surface and that of the surface after the removal of the thermal oxide (corresponding to the Si/SiO2 interface roughness) were observed by means of atomic‐force microscopy. By using metal‐oxide‐semiconductor structured samples, investigations were conducted of the electrical properties induced by surface microroughness, such as the oxide‐trapped charges, Si/SiO2 interface states, neutral oxide‐trap centers, and oxide‐breakdown characteristics. As a result, it was clarified that the neutral oxide traps, as well as the Si/SiO2‐interface states, apparently increase in spite of only a small change in roughness. It was also verified, however, that the oxide‐trapped charges and the oxide breakdown do not change over the scale of roughness change in the present experiments, if contaminants were carefully eliminated from the Si surface. © 1995 American Institute of Physics.