Biofilm formation is one of the most important bacterial virulence factors that plays a key role in infections. In the present study, effects of silver nanoparticles were evaluated in vitro against bacterial biofilm. Ninety bacterial isolates were selected for study. The Congo Red agar, tube and microtitre assays were used for the detection of biofilm. Antimicrobial effects of silver nanoparticles were determined by the Kirby-Bauer and microdilution methods. The microtitre assay was used to study the biofilm inhibition activity. The most common biofilm producing bacteria was Staphylococcus aureus. The power of biofilm production is different among bacteria, and the effect of silver nanoparticles against Escherichia coli was less than Staphylococcus aureus and Pseudomonas aeruginosa. The silver nanoparticles are effective against planctonic and biofilm forms. Because of the importance of biofilm in infectious diseases and the development of drug resistance, silver nanoparticles may be an appropriate way for the control and the prevention of biofilm.