Silica nanoparticles were synthesized by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) using water-in-oil (W/O) microemulsion. Phase behavior experiments were performed to characterize single phase W/O microemulsion region in ternary systems containing ammonia solution, surfactant, and oil, where 3 different types of surfactants and 5 different kinds of oils were used during the experiments. It was found that both surfactant and oil caused a substantial change in microemulsion phase behavior and microemulsion region increased with an increase in the alkyl chain length of an oil. With the information of phase behavior experiments, silica nanoparticles prepared using single phase W/O microemulsion showed that relatively monodisperse particles of spherical shape were obtained. Photographs from transmission electron microscopy showed that average particle size, particle size distribution and number of particles formed were dependent on microemulsion composition, especially contents of ammonia solution solubilized in the microemulsion region.