Spheric silver nanoparticles were synthesized in the presence of polyvinyl pyrrolidone (PVP) as surfactant and sodium carbonate (Na2CO3) as controlled agent by the reduction of silver ions through a microwave-assisted polyol method. The effects of adding amount of Na2CO3 on the silver nanoparticle size were investigated. The morphology, composition, particle size and phase structure of the silver nanoparticles were characterized by SEM, TEM, XPS, XRD and UV-vis. The results show that the average size of silver nanoparticles decreases with the increase of the adding amount of Na2CO3.