Customized microand mesoporous carbons are in high demand for ecofriendly technologies. Reactivation of the well-characterized pitch-based activated carbon fiber (ACF) can provide a clear understanding of the structural mechanism of steam activation, which would be helpful for designing better microand mesoporous carbons. ACFs were reactivated with steam at 973-1173 K. X-ray diffraction and Raman spectroscopy indicated that the stacking number of graphene-like layers of the pore wall decreased with an increase in the reactivation temperature. The average fiber diameter of the ACFs, which was measured via scanning electron microscopy, decreased with the increase in the reactivation temperature. The relationship between the decrease in the fiber diameter and the burn-off suggested that reactivation above 1023 K produced micropores inside the fiber. A deconvolution analysis of the pore-size distribution revealed the variation of the distribution. The peak difference was approximately 0.3 nm, depending on the reactivation temperature. These results indicate that reactivation with steam proceeds via the preferential one-by-one gasification of less-crystalline graphene-like units. (c) 2020 Elsevier Inc. All rights reserved.