Mesoporous silicas having an average pore size of 3.0~7.5 nm have been synthesized via the aqueous Sol-Gel reaction. It was possible to prepare the specimens with a pore size o 3.0~7.5 nm in the pH range of 3.0~5.5. the specimens preparing by the back-titration method which was designed for this study had a larger pore volume than that of preparing by the general titration method in the same reaction conditions. Measurements of the water vapor adsorption-desorption for the specimens comparing with the traditional humidity control materials indicated that they were over 80%. the adsorption-desorption occurred mostly at the relative humidity between 40% and 70%. In this regard, we have concluded that the specimens could be used for a humidity self-control materials.