A previously reported polyimide, 6FDA/DETDA:DABA(3:2), was pyrolyzed under different protocols to produce carbon molecular sieve (CMS) dense film membranes for separation of important gas pairs, including pure gases CO2, CH4, O2, N2 and mixture gases 50%CO2/50% CH4 and 50% C3H6/50% C3H8. This study investigated the effects of pyrolysis temperature, O2 doping, and precrosslinking on the separation performance of 6FDA/DETDA:DABA(3:2) CMS membranes. Comparing to the precursor membranes, separation performance of all 6FDA/DETDA:DABA(3:2) CMS membranes improves significantly with both higher permeability and selectivity. The CMS film pyrolyzed with a novel method, i.e., 800 °C with precrosslinking, shows very attractive separation performance with CO2 and O2 permeability of 4678 Barrer and 683 Barrer, CO2/CH4, O2/N2 selectivity of 71.5 and 8.0. Sorption measurements provide insight into the pore size distributions among these CMS membranes. The effects of these three parameters during pyrolysis on the CMS membranes gas separation performance are summarized and compared, hypotheses regarding the structure changes brought about by changing these parameters are also provided.