Effects of cleaning by H2SO4: H2O2 on thin SiO2 film was investigated. The cleaning increases Fowler-Nordheim currents by about 14%, shifts the dielectric breakdown distribution to lower electric field intensity and degrades TDDB characteristics. These results are due to the oxidation and commensurate roughening of the silicon srface by the cleaning solution. When the cleaning is done at higher temperature and with higher concentration of hydroperoxide, microroughness of silicon surface increases. Therfore, the trade-off between the cleaning effect and the roughening effect of H2SO4: H2O2 should be found out.