Silica aerogels were synthesis by the sol-gel-supercritical drying process using isopropanol as a solvent. Effets of the heat-treatment and the surface modification through propoxylation on the structural reinforcement as well as the surface hydrophobic/hydrophilic characteristics of aerogels were investigated. Silica aerogels synthesized by supercritical drying were hydrophobic but aerogels heat-treated above 20$0^{circ}C$ were transformed to be hydrophilic. In particular it was found that the skeletal structure of aerogels heat-treated at 50$0^{circ}C$ was strong enough not to crack after adsorbing a large amount of water vapor. Hydrophilic aerogels modified by propoxylation at 28$0^{circ}C$ for 20 h were reversed to the hydrophobic form. Transition between hydrophobicity and hydrophilicity was reversible. The hydrophobicvity and the hydrophilicity of silica aerogels were attributed to the Si-Oh bond and the nonpolar C-H bond groups of orgainc species respectively.