This Silica sols were synthesized by alkali hydrolysis of tetraethyl orthosilicate (TEOS). These sols were modified with different percentage of aluminum sol to convert silica from hydrophilic to hydrophobic. The compositions of unmodified and modified silica nanoparticles were studied by X-ray fluorescence (XRF) and confirmed that the aluminum was anchor to the surface of silica nanoparticles. Further analysis by FESEM and Thermo gravimetric analysis (TGA) showed that the amount of modifier added to the silica sols had distinct effect on morphology and thermal stability of silica nanoparticles respectively. X-ray diffraction analysis illustrated that modified silica nanoparticles are amorphous. © (2014) Trans Tech Publications, Switzerland.