Nitrogen adsorption isotherms on untreated activated carbon (U-AC) and plasma-treated activated carbons (PT-ACs) with tetrafluoromethane were investigated. The pore volume was calculated using the Dollimore-Heal method from the adsorption isotherms to elucidate the relationship between plasma treatment time and porosity. The total pore volume of pores with radii less than 95 Angstrom decreased with plasma treatment. However, the pore volume of pores with radii less than 7.5 Angstrom was unchanged. This result was explained by the results of the C constant calculated from the BET equation. The pores with radii of 7.5 to 20.5 Angstrom were particularly influenced by the plasma treatment.