Colloidal silver nanoparticles have been synthesized using silver nitrate solubilized in the water core of one microemulsion as source of silver ions, hydrazine hydrate solubilized in the water core of another microemulsion as reducing agent, dodecane as the oil phase and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as the surfactant. The effect of AgNO3 concentration and the molar ratios of water to AOT (W) in microemulsion on the particle size, the size distribution and the stabilization have been investigated. Transmission electron microscopy (TEM) results show that the silver nanoparticles are all spherical and have a narrow size distribution as well as a good stability. The higher the AgNO3 concentration, the faster the growth rate of the silver particles. The higher W value has been found to give larger particle size and broader size distribution. The resultant nanosilver colloid has low toxicity and high stabilization. As a result, the colloidal nanoparticles need not be separated from the solution and the silver sol can be directly used in antibacterial fields.