The interfacial layer between ambient pressure dried SiO2 aerogel film and GaAs has been examined with emphasis on varying the concentration of aerogel modification agent, trimethylchlorosilane (TMCS) in n-hexane solution. Through surface modification of aerogel, HCl was formed from the reaction between TMCS and -OH bonds of silica aerogel surface, and the GaAs surface could be influenced according to the TMCS concentration in surface modifying solution. When mol concentration of TMCS is 0.066%, the GaAs interface with aerogel was roughly etched from the reflection of surface microstructure of aerogel. However, when the aerogel was modified with 0.05 mol.% of TMCS, uniform shape of the GaAs interface could be obtained and the resultant electrical property, especially leakage current density, showed almost the same behavior when the aerogel was applied to Si system.