The following facts were found by an experimental study of the water evaporation velocity and salt crystallization decay of stone treated with alkoxy silanes: The water evaporation from stone decreases greatly by impregnation treatment with hydrophobic silane (methyl alkoxy silane), though it does not change significantly with a nonhydrophobic one (tetra-alkoxy silane). This is because the hydrophobic layer (resin impregnated surface layer) impedes the migration of the inside water to the surface so that the water evaporates underneath the layer and only the water vapor can go out through it. The evaporation of the water underneath the hydrophobic layer results in crystallization of salt solved in the water there. When the hydrophobic surface layer is thin, it is easily exfoliated by the growing stress of the crystals. This exfoliation of the thin hydrophobic layer of the treated stone precedes the deterioration of an untreated stone. When the hydrophobic layer is thick, it is eventually detached from the stone. However, it occurs much more slowly than the serious deterioration of not only untreated stone but also one treated with inhydrophobic silane. Considering the above facts, it can be said that if a stone absorbs water from backside and the water evaporates out from the surface of the stone continually, such as in the case of a rock-cliff sculpture or the bottom part of a stone construction, treatment with a water repellent giving the stone thin hydrophobic surface layer will make it more susceptible to salt crystallization decay than before. In case hydrophobic layer is quite thick, as for example on a stone treated by deep impregnation with methyl triethoxy silane, its durability against salt crystallization decay can increase greatly, at least as to porous tuff which was used for this study.