Silver nanoparticles with narrow distribution were prepared by a simple route in water in presence of lauric acid as the capping agents, hydrazine hydrate as reducing agents, silver ammonia as silver source. The morphology and crystal structure of as-synthesised silver nanoparticles were analyzed by transmission electron microscopy and X-ray powder diffraction. The mass ratio of lauric acid and silver nitrate, the reacion temperature pay important roles on the morphology and size of the silver nanoparticles. Well-dispersed silver anopartilces with average diameter of 8 nm were obtained when the mass ratio of lauric acid and silver nitrate was 1.2:1 and the reacion temperature was room temperature. It is also confirmed that the silver particels was monodisperse silver nanoparticles by UV-vis spectra .