The in situ small-angle X-ray scattering (SAXS) of water adsorbed on pitch-based activated carbon fibers (ACF) of pore width nl = 0.8 and 1.1 nm were measured at 303 K and different relative pressures. The SAXS spectrum of ACF of w = 1.1 nm in the direction of desorption did not overlap that in the adsorption direction, showing hysteresis. The density fluctuation data from Ornstein-Zernike (OZ) analysis for the SAXS profiles indicated that water molecules form great clusters in the course of adsorption, but desorption proceeds through uniform molecular evaporation.