The stable region of microemulsion of Triton X-100/hexanol/cyclohexane/ water water-in-oil (W/O) system was investigated by using distilled water as aqueous phase. SiO2 nanopowders were prepared by hydrolysis of tetraethoxysilane (TEOS) in the designed reverse microemulsion. The prepared samples were characterized by XRD and TEM. The results showed that the size of particles is within a range of 40~50nm and the aggregation is not evident after heat treatment. The size of particle is minimal and the dispersal is optimal when the ammonia content is 11% in volume.