The adsorption ability of Cr(IV) was greatly enhanced by treating activated carbon(AC) with acid solution, it was much higher than purified AC and activated carbon fiber(ACF). Particularly, it was effective to treat AC with acetic acid(AA) solution. When AC was treated with AA, the absorbance of stretching peak around 1700 cm^-1 due to C=O in carboxyl group was detectably increased. It was therefore considered that the adsorption ability of Cr(IV) was mainly improved by the functional group on the surface. The acid-treated AC will be applied as an adsorbent for controlling the pollutants in air and waste water.