Silica aerogels are nanostructured porous solids with an open pore structure, high surface area, high porosity, low bulk density, and low thermal/electrical conductivity, which have drawn great interest in both science and technology. Generally, silica aerogels are prepared via a sol-gel process with a subsequent drying step, in which a drying condition is usually employed to dry wet silica gels. These days Ambient Pressure Drying (APD) is considered as a practical method of drying. In this experiment, silica aerogels were obtained by two step process, using Tetra Ethyl Ortho Silicate (TEOS) as the precursor and Octadecyltrichlorosilane as the silylation agent under ambient pressure drying condition for the first time and four samples of silica aerogel were successfully synthesized and the BET surface area and porosity of the samples were measured. Optimum sample in terms of high Surface area and porosity was selected and fully characterized. FT-IR was used to validate the surface modification and the results confirmed the surface modification of the wet gel by silylating agent. SEM and TEM was conducted to analyze the morphology and structure, which exhibited the porous and dense network structure and mostly spherical particles. TGA was used to analyze the stability of the aerogel toward increasing temperature and its curve indicated that the aerogel mass, was stable up to 400° C. Nitrogen adsorption/desorption isotherm of the aerogel showed a type IV isotherm with a hysteresis loop, which is the characteristic feature of the mesoporous materials. The strong hydrophobic character is also confirmed by contact angle measurement, which provides an average contact angle of 162°. Key words : silica aerogel, surface modification, hydrophobicity, sol-gel, porosity