Pseudomonas aeruginosa as one of the leading causes of nosocomial infections is a gram negative bacterium and an opportunistic pathogen. Since this bacterium is highly resistant to drugs, several new methods have been investigated to fight against it. Applying nanoparticles of silver is one of the most effective ways to tackle bacterial infections. In this research, the antibacterial properties of nano- silver, with two different sizes, was evaluated for anti-Pseudomonas aeruginosa activity. In this research, the powdered nanoparticles of silver with approximate diameters of 20 nanometers (Pishtazan Inc. Mashad, Iran) and 5 nanometers (Department of Chemistry in Maragheh University) were evaluated for antibacterial activity. The concentration of these nanoparticles was identified by (atomabsorbtion) spectroscopy method in the Faculty of Chemistry in Tabriz University, Iran. Then, the anti-Pseudomonas characteristics of nanoparticles were investigated by MIC (Minimum Inhibitory Concentration), MBC (Minimum Bactericidal Concentration) identification, disc diffusion and well diffusion methods. In this study, was investigated and compared the anti-bacterial effects of nanoparticles of silver on Pseudomonas aeruginosa PAO1 and 20 clinical strains isolated from Imam Reza Hospital in Tabriz, Iran. The findings revealed that the MIC and MBC of 20nm nanoparticles were 625 ppm and 1250 ppm respectively for Pseudomonas aeruginosa PAO1. On the other hand, the MIC and MBC of 5 nm nanoparticles were 75 ppm and 156 ppm respectively for Pseudomonas aeruginosa PAO1. According to these findings, the MIC and MBC identified for clinical Pseudomonas aeruginosa strains under study along with the PAO1 strain failed to show a significant difference. Yet the amount of inhibition for the 20nm nanoparticles in the density of 20000 ppm of clinical Pseudomonas aeruginosa and PAO1 strains was 11 millimeter with the disc diffusion method and 10 millimeters for the well diffusion method with the same concentration. The amount of inhibition of 5nm nanoparticles in the 250- ppm density with disc diffusion and well diffusion methods were 10 and 9 millimeters respectively. Results demonstrated that Pseudomonas aeruginosa is sensitive to nanoparticles of silver. Furthermore, the antibacterial effect of nanoparticles of silver increases as their size decreases. Besides, equality of MIC and MBC in numerous clinical cases reveals that there is no resistance against nanoparticles of silver in drugs resistance pathogenic bacteria.