In this work, polyacrylonitrile (PAN)-based activated carbon fibers (ACFs) were developed by the common processes of stabilization, carbonization, and steam activation. Those fibers were successively subjected to heat treatment in a vacuum at high temperature and reactivation in steam. The changes in specific surface area and surface chemical characteristics were studied. The vacuum treatment reduced the surface area dramatically, while the surface nitrogen decreased after activation in steam and the surface oxygen was lowered upon the vacuum treatment. By curve-fitting the X-ray photoelectron spectroscopy (XPS) of C1s, O1s, and N1s, it was shown that the forms and compositions of the oxygen-containing functionalities seemed not to be affected significantly by the above-mentioned heat-treatment processes, whereas the nitrogen-containing functionalities showed some changes with these treatments.