Abstract: Silica aerogels modified and unmodified with trimethylchlorosilane (TMCS) were prepared by using tetraethoxysilane (TEOS) as the silica source via a two-step sol-gel method combined with supercritical CO₂ drying. The water absorption of two kinds of silica aerogels was measured and the effect of heat treatment temperature on the water absorption of the unmodified silica aerogel was investigated. The silica aerogel modified with trimethylchlorosilane (TMCS) exhibits excellent hydrophobic behavior. For the unmodified silica aerogel, the water absorption increases apparently with heat-treated temperature in the range of 300-500℃, and tends to be almost unchangeable in the range of 500-700℃. After heat-treated at 800℃, the water absorption in-creases remarkably again, whereas after heat-treated at 1 000℃ it decreases significantly due to vitrification of the silica aerogel. The water absorption process of the silica aerogel can be divided into three stages:quick weight gain, slow weight gain and saturation. The relationship between the water absorption of the silica aerogel and its micro and chemical structures was evaluated by differential scanning calorimetry (DSC)/thermogravimetry (TG), infrared spectra (IR), and N₂ absorption techniques.