Gold nanoparticles were prepared by the reduction of sodium tetrachloroaurate in water-in-oil microemulsion, by using polyoxyethylene dodecyl ether, bis 2 ethylhexyl sodium sulfosuccinate, or didodecyl dimethyl ammonium bromide as a surfactant. When the molar ratio of water to surfactant became small, the size of microemulsion was almost monodispersed, and nanoparticles were produced and dispersed well in microemulsion solution. Furthermore, monodispersed nanoparticles were obtained when the number of gold ions per water droplet in microemulsions was above one. The diameter of gold nanoparticles was controllable by the average size of microemulsion. By use of a strong reducing agent such as hydrazine or sodium borohydride, small gold particles could be made.