Abstract: The microstructure of a polyacrylonitrile (PAN) carbon membrane prepared under vacuum or Ar atmosphere was studied by FTIR、XRD and Raman analysis. The change of chemical structure and formation of crystallites during the pyrolysis were also investigated. Results show that intra chain cyclization, dehydration and denitrogenation occur during the pyrolysis. The microstructure of the PAN carbon membrane is transformed from a linear polymer to turbostratic carbon with increasing pyrolysis temperature. Compared with an Ar atmosphere, the pyrolysis of the PAN carbon membrane under vacuum has a low activation energy, i.e. the initial temperature of the pyrolytic reaction of a PAN carbon membrane can be decreased and the pyrolytic reaction rate can be accelerated under vacuum. In addition, the d002、Lc data of PAN carbon membranes prepared under vacuum are smaller while the La data are larger than those obtained under an Ar atmosphere. This indicates that the microstructural regularity of PAN carbon membranes prepared under vacuum are better than for those obtained under Ar, and structural defects and porosity are relatively large in the latter.