Granular activated carbon was treated by oxygen non-thermal plasma to improve its adsorbability for metal ions. The plasma was produced by dielectric barrier discharge under atmospheric pressure. The results of characterization of activated carbon showed that its surface area was decreased, and the concentrations of acidic functional groups at the surface were increased by the treatment. Adsorbability was examined changing the initial pH of the solution containing a metal ion. The saturated adsorption amount of Cu and Zn ion was increased considerably by the treatment. The plasma treatment of activated carbon was concluded as an effective method for improvement of adsorbability.