Well-dispersed silver nanoparticles were prepared by chemical reduction method, with polyvinyl pyrrolidone (PVP) as dispersant. The mechanisms of PVP in the preparation process were discussed through the optical characters of the reaction system. It is indicated that PVP accelerated the reaction between silver ions and glucose, because it stabilized H+. PVP compounded with Ag and protected the silver particles from growing and agglomerating. Silver particles with diameter shorter than 50 nm was protected by the coordination between silver and N in PVP, and for the bigger particles, with the diameter of 500-1000 nm, both N and O coordinated with the silver. (c) 2005 Elsevier B.V. All rights reserved.