Superfine silver nanowires (Ag NWs) with diameter of 40~50 nm and length of 5~10 µm have been synthesized by the self-seeding polyol reduction process using silver nitrate as the silver source, polyvinyl pyrrolidone (PVP) as the structural directing agent, ethylene glycol (EG) as the reducing agent and AgBr as the mediator. The influence of different factors on the growth of Ag NWs was studied. The morphology and crystalline phase of Ag NWs were characterized by the field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD) respectively. The results indicate that the introduction of potassium bromide (KBr) is helpful for growth of Ag NWs with a high aspect ratio, as AgBr colloids formed in the initial stage and the dissociation reaction of AgBr colloids would largely influence the delivering rate of Ag+ ions, which controls the nucleation and growth of the Ag NWs. Finally, the possible growth mechanism of Ag NWs was discussed.