Stable nano-silver colloids with highly pure silver were synthesized by using polyethylene glycol as solvent, stabilizer and reducing agent. The colloidal nanoparticles need not be separated from the solution and the silver sol can be directly used in antibacterial fields. Tannic acid and ascorbic acid were used as co-reducing agents to improve conversion rate of silver ion. Transmission electron microscopy (TEM) analysis showed that stable spherical metal particles 3-50 nm in diameter with a well-crystallized structure were obtained by using tannic acid as co-reducing agent,15-20 nm in diameter were obtained by using ascorbic acid as co-reducing agent. The X-ray diffraction (XRD) study revealed the formation of metallic silver particles crystallized in the face centered cubic (fcc) structure. The antimicrobial test results illustrated that the nano-silver colloids have excellent antimicrobial effects against E.coli and Streptomyces.