To modify the surface properties of fine powders, polymer powders have been oxidized by the active species in an oxygen plasma. A plasma surface modification of polymer powders has been carried out in a fluidized bed reactor. The oxygen functionalities of carbonyl and carboxyl group[C=O, C(O)O-] are formed at the outermost layer of high density polyethylene(HDPE) powder by the oxygen plasma modification without change of bulk property. The effects of operation parameters on the functionalities of the plasma treated high density polyethylene powders have been determined in a fluidized bed reactor. The intensity of oxygen functionalities on the surface of plasma treated HDPE powder increases with increasing treatment time but it remains constant with further treatment time. The oxygen components of plasma treated powder increases linearly with radio frequency power but decreases with oxygen flow rate. Also, the oxygen component increases with increasing the composite parameter, [(W/FM)t], as the total plasma energy up to 6,000GJ·s/kg.