It is well known that the adsorption character of activated carbon is dependent on the specific surface area and pore volume, but the relationship between the surface-chemical structure and the adsorption character has not been studied very often. The purpose of this study is to investigate the effect of the acidic surface functional groups of activated carbon and the adsorption characteristics of low concentration phenol. So three types of activated carbons and four different treatments were introduced to this isotherm experiment. These treatments were nontreatment, 1N HNO_3 treatment, 6N HNO_3 treatment, H_2O_2 treatment. The conclusions of this study are as followings. If the initial concentration of phenol is high as 5㎎/ℓ, the adsorption is dependent on the specific surface area. If the initial concentration of phenol is low as 100㎍/ℓ, the adsorption is dependent on the average pore volume. The acidic surface functional groups prevent the adsorption of phenol molecules to activated carbon. And the adsorbed amount decreases more for HNO_3 treatment than for H_2O_2 treatment and more for concentrated HNO_3 treatment than for dilute HNO_3 treatment.