Abstract: Polyetherimide derived carbon membranes were prepared by pretreating the precursor in air before carbonization at 700℃. The chemical structure, microstructure and gas separation properties of these membranes pretreated at 400, 460 and 480℃ were characterized by FTIR, elemental analysis, XRD and gas permeation tests. The effects of preoxidization on their microstructure and gas separation properties were investigated. Results show that decomposition and oxidationinduced crosslinking reactions occur during the pretreatment. The higher the preoxidization temperature, the higher the degree of decomposition and the degree of crosslinking. After carbonization, the pretreated membranes have an amorphous structure. The differences in the crosslinked structure generated in the pretreatment leads to differences in the micropore structure of the membranes, which can be used to tune gas separation properties. The permeability of all gases investigated exhibits a maximum with a preoxidization temperature at 460℃, for which the O2 permeability is 8.2×10-13(m3(STP) · cm)/(m2 · s · Pa)and the O2/N2 selectivity is 14.1.