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, cyclohexane as the continuous phase, sodium dodecyl sulfate (SDS) as the surfactant, and isoamylalcohol as the cosurfactant. The effects of AgNO3 concentration and the molar ratios of water to SDS (W) on the particle size and size distribution are investigated. UV–vis spectra show that the Ag4+ intermediates formed at early stages of the reaction and then the clusters grow and formed into larger nanoparticles. TEM micrographs confirm that the silver nanoparticles are spherical. Their mean diameters increase from 6.5 nm to 12.1 nm with W values, and the size distribution also increases obviously with the W values. By absorption of SDS molecules from microemulsions, the as-prepared nanoparticles are stable without aggregation. However, the stabilization achieved by absorption of SDS, i.e., the protection of Ag nanoparticles by SDS, is relatively weak.