The attractive force acting at a carboxyl group of a fatty acid/oxidized aluminum interface was precisely determined by atomic force microscopy (AFM). The correlation of the attractive force with the sizing behavior of paper sheets sized by a fatty acid-aluminum sulfate (alum) system was investigated. The surfaces of gold-coated AFM probes were modified with alkylthiol terminating methyl or carboxyl groups through self-assembling chemisorption. Pure cellulose film and oxidized aluminum plate were used as model substrates of cellulosic pulp fibers and aluminum components, respectively, in the sized paper sheet. A strong attractive force was detected at a carboxyl-terminated AFM probe/aluminum oxide interface both in air and water, while very weak interaction was found at the other pairs of probe/ substrates. In the fatty acid-alum systems, it was observed that such a combination was indispensable for good sizing appearance of not only organic pulp fiber sheet, but also inorganic glass filter. Consequently, the specific attractive interaction between carboxyl termini of acid sizing agents and oxidized aluminum components in the alum-treated fiber matrix should play a significant role in the water-repellent properties of porous fiber-network materials.