Gold nanoparticles were prepared in W/O microemulsions of AOT/heptane/HAuCl4·3H2O system using N2H4·H2O as the reducing regent. These particles were characterized by UV-vis spectrum, TEM and FT-IR. The influences of the molar ratio of water to surfactant (ω=nH2O/nAOT) and the concentration of tetrachloroauric acid on the particle size were investigated. The results show that the average particle size of the prepared gold nanoparticles is from 4 nm to 12 nm, and their distribution is narrow. The relation between ω and average particles size is linearity. On the other hand, the influence of tetrachloroauric acid concentration is complex. As the concentration increases, the particle size decreases firstly and increases subsequently. FT-IR spectra indicate that the surfactant molecules are strongly absorbed on the surface of gold nanoparticles through a coordination bond between the gold atom and the sulfonic group of AOT molecules, so the surfactant AOT has a good protection effect.