Dense membranes were prepared from poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) by using 1,1,2-trichloroethylene (TCE) and carbon disulfide as solvents. The membranes so prepared were characterized by the combination of plasma etching and atomic force microscopy (AFM). The membranes were also characterized by permeation of oxygen and nitrogen gases before and after plasma etching. It was found that the morphology of the top surface layer of unetched PPO-CS2 was different from that of top surface of PPO-TCE membrane. The surface of PPO-CS2 membrane was covered by ＇supernodular aggregates＇, whereas that of PPO-TCE membrane was covered by nodular aggregates. It was found also that PPO-CS2 membrane exhibited higher permeability than PPO-TCE membrane. The performance of the membrane in terms of selectivity and permeability could be improved by plasma etching for a short time. Discussions were made on the morphological change to the depth direction based on AFM pictures.