In recent years, application of medicinal plants increase in the treatment of infections rather than antimicrobial drugs. The aim of this study was to compare the effects of silver nanoparticles and alcoholic extract of Amaranthus retroflexus on some pathogenic bacteria. The alcoholic extract of this plant was prepared and the effect of concentrations of 50 to 400 mg/ml of this extract and concentrations of 10 to 80 mg/ml of silver nanoparticles by well diffusion method on S. aureus, B. cereus, E coli and P aeruginosa were evaluated. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined on bacteria by dilution method. The results showed that the inhibition effect of Amaranthus retroflexus on gram-positive bacteria is more than gram negative bacteria while the inhibitory effect of silver nanoparticles on gram-negative bacteria is more than gram-positive bacteria. The results of this study showed that the alcoholic extract of Amaranthus retroflexus had an antibacterial properties that can be considered as an adjunct to further studies for in vivo conditions and replacement with common chemical agents in the treatment of infections.