Adsorption isotherms of water vapor on active carbons are interpreted in terms of the fractal approach. The linear range of plots in the steep rise of the adsorption branch is the process of the cluster formation of water on the seed water molecules adsorbed on the primary adsorption centers, mainly the oxygen-surface complexes, with the fractal dimension D = 1.64-1.67 which is equivalent to that for the diffusion-limited aggregation (DLA) model with D approximate to 1.66. Also, it is shown that the two-dimensional characteristics of micropore structures are reflected in the adsorption branch and the three-dimensional ones are in the desorption branch, respectively, by the analyses of the D and the average micropore width L based on the characteristic adsorption amounts at the beginning and the ending points of the steeply rising and falling linear ranges on both branches.