Hydrophobic SiO2 aerogels were produced by the surface modification of alcogels prepared via sol-gel process using polyethoxydisiloxanes (E-40) as the precursor and followed by ethanol supercritical drying. The structure of the modified silica aerogels was a silica matrix produced by hydrolysis and condensation of polyethoxydisiloxanes. Si-CH3 was modified on the surface of the silica matrix. The pore size decreased from 23.1nm to 18.2nm, the specific surface area increased from 477 m2*g-1 to 563 m2*g-1 and the water vapor adsorption decreased from 0.04 to 0.0012(weight ratio) after the surface modification of silica aerogels. The existence of Si-CH3 was observed using infrared spectra.