Silica aerogels have interesting physical properties such as hydrophobicity and low thermal conductivity are advantageous for a wide variety of actual applications, such as super thermal insulators. Poor mechanical properties prevented to use silica aerogel directly. In this study, nano silica aerogel and its composite with cotton were synthesized using a water glass precursor by ambient pressure drying method .To modify silica aerogel use trimethyl chlorosilane (TMCS) diluted in n-hexane (1:5 volume ratio). By adding TMCS, –OR groups in silica aerogel structure replaced –OH and silica aerogel became hydrophobic. Thermal conductivity and hydrophobicity of synthesized samples were measured by needle probe method and contact angle meter respectively. Pure silica aerogel contact angel is 166 o and it shows silica aerogel is super hydrophobic. Silica aerogel added to cotton structure in 20 %, 40 %, 60 % and 80 % wt. Silica aerogel particles attached cotton fibers and changed hydrophobicity properties. Contact angels of composites are 115°, 120°, 128° and 129° respectively. Cotton is hydrophilic and its ability to absorb moisture causes decreasing contact angel in composites. Silica aerogel and cotton thermal conductivities are 0.0134 Wm-1K-1and 0.0308 Wm-1K-1respectively. Having low thermal conductivity, pure silica aerogel is known as super thermal. Thermal conductivity of composites was measured 0.0251, 0.0227, 0.0216 and 0.0171 Wm-1K-1 respectively. By increasing silica aerogel ratio, thermal conductivity decreased. In silica aerogel- cotton composite improved structural strength and the thermal conductivity and hydrophobicity were acceptable.