Abstract: The micropore structure of activated carbon fibres (ACFs) can be modified by heat treatment. Pitch-based ACFs were heat treated at 1173 K and the effects of heat treatment on adsorption properties, pore size distribution, micropore structure and surface chemistry were studied, using (77 K) N2 adsorption, density functional theory (DFT), X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). Results show that although high temperature treatment decreases the BET surface area, the micropore size is much more uniform and narrow down to the range 0.5 to 1.0 nm and therefore the iodine adsorption capacity is obviously enhanced. XRD shows that the carbon layers in the resultant ACF are still turbostratic but d002 decreased due to the high temperature treatment. XPS analyses indicate that surface alkalescence of ACF decreases with the reduction of the surface oxygen-containing group C-OH.