We succeeded in fabricating new carbon molecular sieve (CMS) membranes as hollow fibers using poly(phenylene oxide) (PPO) and its functionalized derivatives (R-PPO). Gas transport properties were measured for He, H-2, CO2, O-2, and N-2. PPO CMS membranes exhibited higher performances than those of polymeric precursors. The highest performance was attained by trimethylsilyl-PPO (TMSPPO) CMS membrane pyrolyzed at 923 K, of which O-2 permeability was 125 Barrer and O-2/N-2 permselectivity was 10.0 at 298 K.