Low dielectric constant thin film of nanoporous silica synthesized by sol-gel was deposited on Si(100) substrate by spin coating. By detecting —CH3 substituted for —OH species, which can avoid the destruction of network, surface modification was identified by Fourier transform infrared spectroscope. The hole size was about 70-80 nm observed from scanning electron microscope. By adjusting the pH value of the sol, we found that the gel time increased with the decrease o f the pH value of the sol. When heating the modified film at different temperatu res (60-400 ℃)， we can obtain the lowest dielectric constant 2.05 at 300 ℃ .