SiO2 xerogel thin film with a low dielectric constant was successfully prepared by a two-step acid-base catalyst procedure and successive surface modification with trimethylchlorosilane (TMCS). Only 15% porosity could be obtained without surface modification but with surface modification the porosity increased to 50%. These porosity values correspond to measured dielectric constants of 3.95 and 2.45, respectively. The low dielectric constant was revealed to depend mainly on the porous structure of xerogel thin film obtained with surface modification. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses were carried out to evaluate the effect of surface modification which induces the changes of surface coverage from -OC2H5 and -OH to -CH3. Rutherford backscattering spectrometry analysis gave the porosity of SiO2 xerogel thin film as 47.5%. The porosity estimation using refractive index obtained from Ellipsometry work was 43.2% and agrees well with RBS analysis. The network structure of SiO2 xerogel was also evaluated with scanning electron microscopy and transmission electron microscopy.