Recent studies have demonstrated that significantly low sheet resistance (Rs) (<100 Ω/sq; comparable to ITO) were achieved in single-walled carbon nanotube (SWCNT) films treated with HNO3 followed by thionyl chloride. Here we show that H2SO4 can effectively reduce the Rs of SWCNT electrodes. H2SO4 treatment generates defects (COOH and SO3H functionalities) on SWCNTs and the produced chemical functionalities are beneficial for enhancing the electrical conductivity in SWCNT electrodes. It is plausible that the H2SO4p-dopes the SWCNTs and the attachment of chemical functionalities helps to stabilize p-doping owing to their electron-deficient property.