Colloidal silver nanoparticles were synthesized in AOT microemulsions. The effects of the precursors' concentration and the molar ratios of water to AOT on the particle size and size distribution were investigated. UV-vis spectra showed that the Ag+4 intermediates formed at low N2H4 concentration. TEM micrographs confirmed that the silver nanoparticles are all spherical with mean diameters in the range 2-5 nm and have a narrow size distribution. Not only the particle size but also the size distribution was increasing with the W value. The silver colloid has favorable stability and can be preserved for a long time without precipitation.