This work presents the different effects of steam and carbon dioxide activation on the microstructure of an oxidized polyacrylonitrile (PAN) fabric. An investigation was conducted on a series of carbonized fabrics and two series of activated carbon fabrics. The fabrics were activated by steam and carbon dioxide using heat-treatment temperatures of 900-1100 degreesC. Steam and carbon dioxide developed the microstructure initially present in the PAN-based activated carbon fabrics, but with different effects. These fabrics in the form of fabric and powder were examined by X-ray diffraction and Raman spectrometry. This study indicated that carbon dioxide only reacted with the crystalline edges or the irregular carbon on the fiber surface and that the inside structure of the fibers was not greatly affected. When the fabrics were activated using steam, water molecules reacted not only on the fiber surface but also with the carbon at the crystal edge and/or the nonregular carbon in the fibers, which led to communicating pore structures on the surface and in the inner portions of the fiber. This activation also promoted the denitrogenation reactions. Because of these structures and reactions, the activated carbon fabrics, which were activated by steam, had the highest stacking height for carbon layer planes (L-c), the highest number of layer planes (L-c/d(002)), the highest oxygen content, the largest crystal size (L-a), and the highest density over the other samples.