The experimental results of thermal process on the microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths are reported and discussed. With sodium silicate as precursor, ethanol/hexamethyldisiloxane/hydrochloric acid as surface modification agent, the crack-free and high hydrophobic silica aerogel monoliths was obtained possessing the properties as low density (0.096 g/cm3), high surface area (651 m2/g), high hydrophobicity (~147°) and low thermal conductivity (0.0217 Wm/K). Silica aerogels maintained hydrophobic behavior up to 430 °C. After a thermal process changing from room temperature to 300 °C, the hydrophobicity remained unchanged (~128°), of which the porosity was 95.69% and specific density about 0.094 g/cm3. After high temperature treatment (300–500 °C), the density of final product decreased from 0.094 to 0.089 g/cm3 and porosity increased to 96.33%. With surface area of 466 m2/g, porosity of 91.21% and density about 0.113 g/cm3, silica aerogels were at a good state at 800 °C. Thermal conductivities at desired temperatures were analyzed by the transient plane heat source method. Thermal conductivity coefficients of silica aerogel monoliths changed from 0.0217 to 0.0981 Wm/K as temperature increased to 800 °C, revealed an excellent heat insulation effect during thermal process.