Ultra-low density silica aerogel is synthesized through a two-step sol-gel processing and supercritical fluid drying. However, in existing two-step sol-gel processing, a wet gel is obtained by complicated steps : addition of alcohol as a cosolvent to the mutually immiscible TMOS and water solution; hydrolysis of under acidic solution(i.e., HCI) : separation of alcohol by distillation; dilution of the mother liquor by a diluent such as acetone; and polymerisation to obtain wet gel. In the present study, a new simplified two-step sol-gel processing is proposed. First, through a phase behavior study for the ternary solution containing TMOS, water and acid, a condition which becomes a homogeneous single phase was searched. With this solution, hydrolysis is performed. Thus, the separation step of alcohol can be eliminated without adding alcohol. The wet gel which was obtained by the new process was directly dried to obtain the aerogel by SC CO₂ at 40℃ and 8 MPa. In general, the aerogel obtained by the low-temperature supercritical carbon dioxide drying is extremely hydrophilic and easily destructed by moisture. However, by a methoxylation of the surface of the aerogel with methanol at 180℃, hydrophobic aerogel can be made. For most cases, the density of the aerogels obtained in this work stays below 0.03 g/cm³.