Background and Objective: Nano-sized particles at scale of 1 to 100 nm, called nanoparticles. In addition, the composition and structure of materials is also one of the factors influencing the material properties. With the advent of nanotechnology and due to increasing antimicrobial properties of nanoscale silver it can also be used in the fight against various human pathogens. This study was carried out to evaluate the antibacterial effect of silver nanoparticles synthesized by green method against the standard strains Escherichia coli k12 and Escherichia coli 25922. Methods: In this descriptive study, silver nanoparticles were synthesized using Prosopis farcta seed exudates and analyzed by UV visible spectrophotometer, X-ray diffraction and transmission electron microscopy. Antibacterial effect of silver nanoparticles was evaluated using broth macro-dilution method. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of silver nanoparticles was determined on the standard strains of Escherichia coli k12 and Escherichia coli ATCC 25922. Results: Transmission electron microscopy showed nanoparticles with diameters in the range between 5-35 nm with a maximum frequency range in 20-25 nm. The minimum inhibitory concentrations of bacteria, of E. coli k12 and E. coli 25922 respectively, were 1.56 and 0.39 µg/ml (ppm) and minimum bactericidal concentrations of 3.12 and 0.78 µg/ml wiring (ppm). Conclusion: Biological synthesis using P. farcta seed is a inexpensive, method and require no energy. Due to the strong antibacterial activity of silver nanoparticles, can be a suitable alternative for disinfectants, disinfection and control of pathogens.