Silica nanopaticles were synthesized by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate(TEOS) using water in oil microemulsions. Phase behavior experiments were performed to characterize one phase water-in-oil type microemulsion region in ternary systems containing water(NH3), surfactant, and oil. Of particular interest was the effect of surfactant and oil on formation of one phase microemulsion where seven different types of surfactants and five different oils such as n-decane, n-heptane, cyclohexane, toluene, and isooctane were used Both surfactant and oil caused a substantial change in microemulsion phase behavior and thus SiO2 nanoparticle characteristics made from one phase water-in-oil microemulsion. Photographs from transmission electron microscopy showed that average particle size, particle size distribution and number of particles formed were depend on microemulsion composition, especially water contents solubilized in the microemulsion region.