Abstarct: The colloid nano silver was prepared by chemical reduction method from reaction of silver nitrate with sodium borohydride in presence of trisodium citrate and polyvinyl pyrrolidone. The particle size of the sample is determined by UV spectrum and TEM photograph methods. The antimicrobial properties of nano silver are investigated by two methods: MIC and MBC against gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus). In the MBC method, nano silver is completely destroyed Escherichia coli bacteria with concentration 50 PPM and Pseudomonas aeruginosa, Bacillus subtilis and Staphylococcus aureus of bacteria with 25 PPM. Also, in MIC method, the concentration of 15PPM nano silver can stoped the growth of all 4 types of bacteria.