Surface modification is a key process for the preparation of porous SiO2 xerogel film by ambient pressure drying. The microstructure and surface terminal bonds which determine the properties of the film are closely related with the surface modification. Two kinds of pre-aged film were prepared to investigate the effect of wet-gel microstructure on the surface modification of film. The concentration of surface modifying agent (tetramethylchlorosilane; TMCS) in solvent (n-hexane) was also varied. As concentration of TMCS increased, surface morphology was obtained from a two-dimensional structure to a three-dimensional network structure with pre-aged film for 1 h at 70 degreesC. This is due to the increase in methyl groups as surface terminal bonds and the suppression of condensation reaction. Variation in film thickness, porosity, dielectric constants, and I-E characteristic according to the TMCS concentration in n-hexane modifying solution was explained in the view-point of the nature of wet-gel structure. However, because of the additional increase in the strength of network and the effective exchange of surface terminal -OH groups with -OSi(CH3)(3) groups even with low concentration of TMCS, there were no serious changes in the properties of SiO2 xerogel film pre-aged for 12 h at 70 degreesC.